Derivative Instruments (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Derivatives, Fair Value [Line Items]
Total gain (loss) to be reclassified during next 12 months	$ (36)
Margin cash collateral received from counterparties that was not offset against derivative assets	26	$ 27
Margin cash collateral provided to counterparties that was not offset against derivative assets or liabilities	150	$ 116
Reclassification out of Accumulated Other Comprehensive Income [Member] | Interest Expense [Member]
Derivatives, Fair Value [Line Items]
Reclassification out of AOCI into interest expense, before tax	14
Reclassification out of AOCI into interest expense, net of tax	$ 9