Income Taxes (Income Tax Expense Reconciled to Federal Income Tax Rate) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income before income taxes	$ 830	$ 741	$ 768
Income tax expense at 35% statutory rate	291	260	269
Production tax credits	(22)	(15)	(5)
Investment tax credits	(5)	(5)	(6)
Depreciation	3	3	3
AFUDC - Equity	(7)	(5)	(4)
Employee Stock Ownership Plan dividends	(3)	(2)	(3)
Domestic production activities deduction	(2)	(18)	(16)
State and other income taxes, net of federal benefit	43	41	40
Other, net	0	(5)	4
Income tax expense	$ 298us-gaap_IncomeTaxExpenseBenefit	$ 254us-gaap_IncomeTaxExpenseBenefit	$ 282us-gaap_IncomeTaxExpenseBenefit
Effective income tax rate (as percent)	35.90%	34.30%	36.70%

Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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This element represents the income or loss from continuing operations attributable to the economic entity which may also be defined as revenue less expenses from ongoing operations, after income or loss from equity method investments, but before income taxes, extraordinary items, and noncontrolling interest.

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Amount of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Amount of the difference between reported income tax expense (benefit) and the expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to deduction for qualified production activity.

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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Amount of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to tax exempt income, equity in earnings (loss) of an unconsolidated subsidiary, minority noncontrolling interest income (loss), tax holiday, disposition of a business, disposition of an asset, repatriation of foreign earnings, repatriation of foreign earnings jobs creation act of 2004, increase (decrease) in enacted tax rate, prior year income taxes, increase (decrease) in deferred tax asset valuation allowance, and other adjustments.

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