Derivative Instruments (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosure [Abstract]
Total loss to be reclassified during next 12 months	$ 22
Margin cash collateral received from counterparties that was not offset against derivative assets	8	$ 10
Margin cash collateral provided to counterparties that was not offset against derivative assets or liabilities	$ 188	$ 40