Derivative Instruments (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Derivative [Line Items]
Cash Flow Hedge Gain (Loss) to be Reclassified within Twelve Months	$ (25)
Derivative, Collateral, Obligation to Return Cash	10	$ 5
Margin Cash Collateral Not Netted Against Derivative Liabilities	40	129
Interest Expense [Member] | Reclassification out of Accumulated Other Comprehensive Income [Member]
Derivative [Line Items]
Reclassification from AOCI, Current Period, before Tax, Attributable to Parent	2	18
Reclassification from AOCI, Current Period, Net of Tax, Attributable to Parent	$ 1	$ 11