Income Taxes (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Income Tax Disclosure [Abstract]
Effective income tax rate (as a percent)	37.00%	26.00%	33.00%	35.00%
Production tax credits	$ 49	$ 66	$ 98	$ 125
Deferred income tax benefit associated with convertible investment tax credits	10	21	22	33
Income Tax Charge Associated with Canadian Assets in NEP Portfolio	45			45
Valuation Allowances and Reserves, Charged to Cost and Expense				132
Impairment of Long-Lived Assets Held-for-use	0	0	0	300
Impairment Of Long Lived Assets Held For Use After Tax				$ 342