Amendment Description Amendment Flag Current Fiscal Year End Date Document Period End Date Document Type Entity [Domain] Entity Well-known Seasoned Issuer Entity Voluntary Filers Entity Current Reporting Status Entity Filer Category Entity Public Float Entity Registrant Name Entity Central Index Key Entity Common Stock, Shares Outstanding Document Fiscal Year Focus Document Fiscal Period Focus Legal Entity [Axis] Accounts Receivable, Net, Current Receivables, net of allowance for uncollectible accounts of $6 and $9 Receivables, net of allowance for uncollectible accounts of $3 and $3 Accumulated Other Comprehensive Income (Loss), Net of Tax Total Accumulated Depreciation, Depletion and Amortization, Property, Plant, and Equipment Nonutility property, accumulated depreciation Amortization Investment Tax Credit Total investment tax credits Allowance for Doubtful Accounts Receivable, Current Receivables, allowance for uncollectible accounts Uncollectible accounts Allowance for Doubtful Accounts [Member] Asset retirement obligations and related funding Asset Retirement Obligation Costs [Member] Asset Retirement Obligation, Accretion Expense Accretion expense Asset Retirement Obligation, Liabilities Incurred Liabilities incurred Asset Retirement Obligation, Liabilities Settled Liabilities settled Asset Retirement Obligations, Noncurrent Asset retirement obligations Asset Retirement Obligation, Revision of Estimate Revisions in estimated cash flows Asset Retirement Obligation Beginning balance Ending Balance CONSOLIDATED BALANCE SHEETS Cash and Cash Equivalents, at Carrying Value Cash and Cash Equivalents, January 1 Cash and Cash Equivalents, December 31 Cash and cash equivalents Interest Paid, Net Cash paid for-Interest (net of capitalized interest of $7, $9 and $23) Cash paid for-Interest (net of capitalized interest of $7, $9 and $22) Increase (Decrease) in Inventories Inventories Increase (Decrease) in Prepaid, Deferred Expense and Other Assets Prepayments and other Prepayments Increase (Decrease) in Receivables Receivables Proceeds from (Repayments of) Short-term Debt Short-term borrowings, net Increase (Decrease) in Accounts Payable Accounts payable Commercial Paper [Member] Commercial paper Commercial Paper. Outstanding commercial paper (270 or fewer days) Commitments and Contingencies Disclosure [Text Block] COMMITMENTS AND CONTINGENCIES Common Stock, Shares Authorized Authorized shares of common stock Comprehensive Income (Loss), Net of Tax, Attributable to Parent Total Comprehensive Income (Loss) Corporate debt securities Corporate Debt Securities [Member] Fuel Costs Cost of Purchased Oil and Gas Gas purchased for resale Cost of Purchased Power Current Income Tax Expense (Benefit) Total current taxes Current Liabilities: Liabilities, Current [Abstract] Liabilities, Current Total Current Liabilities Customer Advances and Deposits, Current Customer deposits and customer prepayments Debt Disclosure [Text Block] LONG-TERM AND SHORT-TERM DEBT Total debt Long-term Debt, Gross Debt Instrument, Decrease, Repayments Retirement of debt Debt Instrument, Face Amount Face value of Industrial Revenue Bonds issued, proceeds of which were availed as loan Debt Instrument, Increase, Additional Borrowings Face amount of debt issued Debt Instrument, Interest Rate at Period End Weighted average interest rate (as a percent) Debt Instrument, Interest Rate, Stated Percentage Stated interest rate (as a percent) Debt Instrument, Unamortized Discount Unamortized discount Debt Instruments [Abstract] Long-term Debt Debt Instrument [Axis] Debt Instrument [Line Items] Long-term Debt and Liquidity Long-term debt Schedule of Long-term Debt Instruments [Table] Assets held in trust, net-nuclear decommissioning Decommissioning Fund Investments Assets held in trust, net - nuclear decommissioning Decommissioning Liability, Noncurrent Nuclear plant decommissioning Deferred Compensation Arrangement with Individual, Compensation Expense Compensation expenses recognized resulting from fair value adjustments of performance awards Deferred Compensation Arrangements [Abstract] Liability Awards Deferred Income Tax Expense (Benefit) Deferred income taxes, net Total deferred taxes Deferred Tax Assets (Liabilities), Net Deferred Tax Assets Net deferred tax liability Net deferred tax liability Deferred Tax Assets, Net Total deferred tax assets Deferred Tax Assets, Other Other Deferred Tax Assets, Tax Deferred Expense, Compensation and Benefits Deferred compensation Deferred Tax Assets, Tax Deferred Expense, Reserves and Accruals Nondeductible reserves Deferred Tax Liabilities [Abstract] Deferred tax liabilities: Deferred Tax Liabilities Total deferred tax liabilities Deferred Tax Liabilities, Other Other Accumulated deferred income taxes Deferred Income Tax Charges [Member] Defined Contribution Pension and Other Postretirement Plans Disclosure [Abstract] Stock Purchase Saving Plan Defined Contribution Plan, Cost Recognized Matching employer's contribution to the plan Depreciation, Amortization and Accretion, Net Depreciation and amortization Derivative Assets Assets Derivative, by Nature [Axis] Derivative Instruments and Hedging Activities Disclosure [Text Block] DERIVATIVE FINANCIAL INSTRUMENTS Derivatives not designated as hedging instruments - gain (loss) recognized in income, classified in gas purchased for resale Derivative Instruments Not Designated as Hedging Instruments, Gain (Loss), Net Derivative [Line Items] Natural Gas Derivative Contracts and Interest Rate Swaps Outstanding Fair value of energy-related derivatives and interest rate derivatives Derivative [Table] Guarantor Obligations, Maximum Exposure, Undiscounted Maximum future payments of guarantees, undiscounted Earnings Per Share, Diluted Diluted Earnings Per Share of Common Stock (in dollars per share) Diluted earnings per share (in dollars per share) Effect of Fair Value Hedges on Statement of Income Effect of Fair Value Hedges on Results of Operations [Abstract] Effective Income Tax Rate Reconciliation, at Federal Statutory Income Tax Rate Statutory federal income tax rate (as a percent) Electric Revenue Electric Employee Service Share-based Compensation, Allocation of Recognized Period Costs, Capitalized Amount Capitalized compensation costs Allocated Share-based Compensation Expense Compensation expense recognized in income Employee Service Share-based Compensation, Tax Benefit from Compensation Expense Tax benefit related to stock-based compensation recognized Energy Related Inventory Fuel Environmental remediation costs Environmental Restoration Costs [Member] Equity Method Investment, Ownership Percentage Percentage of ownership interest (as a percent) Equity Securities [Member] Common stock Assets Held-in-trust CONSOLIDATED STATEMENTS OF INCOME INCOME TAXES Income Tax Disclosure [Text Block] Income Tax Reconciliation, Deductions, Qualified Production Activities Domestic production activities deduction Income Tax Reconciliation, Other Adjustments Other differences, net Income Tax Reconciliation, State and Local Income Taxes State income taxes (less federal income tax effect) Income Taxes Paid, Net Cash paid for-Income taxes Increase (Decrease) in Other Operating Assets Changes in other assets Interest Expense Interest charges, net of allowance for borrowed funds used during construction of $7, $10 and $23 Interest charges, net of allowance for borrowed funds used during construction of $7, $10 and $22 Interest Expense Accrued interest expense Unrecognized Tax Benefits, Interest on Income Taxes Expense Accrued interest expense related to unrecognized tax benefits Fair Value of Interest Rate Swaps Interest Rate Swap [Member] Inventories (at average cost): Inventory, Net [Abstract] Other Inventory, Supplies Materials and supplies Jointly Owned Utility Plant, Proportionate Ownership Share Percent owned (as a percent) Nuclear electric generation units Jointly Owned Nuclear Power Plant [Member] Jointly Owned Utility Plant, Ownership Amount of Construction Work in Progress Construction work in progress Jointly Owned Utility Plant [Axis] Jointly Owned Utility Plant, Gross Ownership Amount of Plant in Service Plant in service Jointly Owned Utility Plant, Ownership Amount of Plant Accumulated Depreciation Accumulated depreciation Junior Subordinated Notes (unsecured) Junior Subordinated Debt [Member] Capitalization and Liabilities Liabilities and Equity [Abstract] Liabilities and Equity Total Line of Credit Facility, Amount Outstanding LOC advances Line of Credit Facility, Interest Rate at Period End LOC advances, weighted average interest rate (as a percent) Line of Credit Facility, Maximum Borrowing Capacity Committed long-term line of credit, Total Line of Credit Facility, Remaining Borrowing Capacity Available amount under lines of credit Line of Credit Facility [Abstract] Lines of credit: Line of Credit Facility [Axis] Line of credit Line of Credit [Member] Long-term Debt, Current Maturities Current portion of long-term debt Current maturities of long-term debt Long-term Debt, Maturities, Repayments of Principal in Next Twelve Months 2012 Long-term Debt, Maturities, Repayments of Principal in Year Five 2016 Long-term Debt, Maturities, Repayments of Principal in Year Four 2015 Long-term Debt, Maturities, Repayments of Principal in Year Three 2014 Long-term Debt, Maturities, Repayments of Principal in Year Two 2013 Loss Contingencies [Line Items] Commitments and contingencies Medium-Term Notes (unsecured)