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Income Taxes Components of Income Tax Expense (Benefit) (Details) (USD $)
In Millions, unless otherwise specified
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Income Tax Disclosure [Abstract]      
Current federal $ 2us-gaap_CurrentFederalTaxExpenseBenefit $ 54us-gaap_CurrentFederalTaxExpenseBenefit $ (112)us-gaap_CurrentFederalTaxExpenseBenefit
Current state 10us-gaap_CurrentStateAndLocalTaxExpenseBenefit 13us-gaap_CurrentStateAndLocalTaxExpenseBenefit (3)us-gaap_CurrentStateAndLocalTaxExpenseBenefit
Total current income tax expense (benefit) 12us-gaap_CurrentIncomeTaxExpenseBenefit 67us-gaap_CurrentIncomeTaxExpenseBenefit (115)us-gaap_CurrentIncomeTaxExpenseBenefit
Deferred federal 260us-gaap_DeferredFederalIncomeTaxExpenseBenefit 204us-gaap_DeferredFederalIncomeTaxExpenseBenefit 283us-gaap_DeferredFederalIncomeTaxExpenseBenefit
Deferred state 43us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit 29us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit 33us-gaap_DeferredStateAndLocalIncomeTaxExpenseBenefit
Total deferred income tax expense 303us-gaap_DeferredIncomeTaxExpenseBenefit 233us-gaap_DeferredIncomeTaxExpenseBenefit 316us-gaap_DeferredIncomeTaxExpenseBenefit
Investment tax credits (6)us-gaap_OtherTaxExpenseBenefit (3)us-gaap_OtherTaxExpenseBenefit (4)us-gaap_OtherTaxExpenseBenefit
Total income tax expense $ 309us-gaap_IncomeTaxExpenseBenefit $ 297us-gaap_IncomeTaxExpenseBenefit $ 197us-gaap_IncomeTaxExpenseBenefit