Fair Value of Financial Instruments Unobservable Input Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance as of the beginning of the period	$ 153	$ 158	$ 119	$ 119
Net realized and unrealized losses (gains)/losses	(1)	0	34	40
Transfers out of Level 3 to Level 2	1	2	0	1
Balance as of the end of the period	$ 153	$ 160	$ 153	$ 160