Fair Value of Financial Instruments Unobservable Input Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance as of the beginning of the period	$ 144	$ 131	$ 119	$ 119
Net realized and unrealized losses (gains)	9	28	35	40
Transfers out of Level 3 to Level 2	0	(1)	(1)	1
Balance as of the end of the period	$ 153	$ 158	$ 153	$ 158