Derivative Instruments (Details) - USD ($) $ in Millions	9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Total loss to be reclassified during next 12 months	$ 27
Margin cash collateral received from counterparties that was not offset against derivative assets	13		$ 5
Margin cash collateral provided to counterparties that was not offset against derivative assets or liabilities	177		$ 129
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	2	$ 17
Reclassification out of AOCI into interest expense, net of tax	$ 1	$ 10