Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Income Tax Disclosure [Abstract]
Effective income tax rate (as a percent)	28.00%	37.00%	29.00%	33.00%
Production tax credits	$ 37	$ 49	$ 75	$ 98
Deferred income tax benefit associated with convertible investment tax credits	$ 34	13	$ 52	33
Operating Loss Carryforwards [Line Items]
Income Tax Charge Associated with Canadian Assets in NEP Portfolio		$ 45		$ 45