Income Taxes (Reconciliation of Income Tax Expense to the Statutory 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	$ 1,275	$ 922	$ 960
Income tax expense at 35% statutory rate	446	323	336
Production tax credits	(119)	(68)	(49)
Investment tax credits	(6)	(6)	(6)
Depreciation	(4)	(4)	(4)
AFUDC - Equity	(7)	(5)	(4)
Employee Stock Ownership Plan dividends	(4)	(4)	(4)
Domestic production activities deduction	0	(14)	(14)
State and local income taxes, net of federal benefit	51	37	37
Enactment of New York Corporate Income Tax Legislation, net of federal benefit	8	0	0
Other, net	(1)	(5)	(6)
Income tax expense	$ 364	$ 254	$ 286
Effective income tax rate (as percent)	28.50%	27.50%	29.80%