Income Taxes (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Income Tax Disclosure [Abstract]
Effective income tax rate (as a percent)	32.00%	30.00%	30.00%	31.00%
Production tax credits	$ 29	$ 34	$ 105	$ 132
Deferred income tax benefit associated with convertible investment tax credits	$ 16	$ 37	$ 67	69
Income Tax Charge Associated with Canadian Assets in NEP Portfolio				$ 45