Derivative Instruments (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Dec. 31, 2016
Derivatives, Fair Value [Line Items]
Total loss to be reclassified during next 12 months	$ 29
Margin cash collateral received from counterparties that was not offset against derivative assets	11		$ 5
Margin cash collateral provided to counterparties that was not offset against derivative assets or liabilities	160		$ 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	$ 15
Reclassification out of AOCI into interest expense, net of tax	$ 1	$ 9