Fair Value of Financial Instruments Unobservable Input Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance as of the beginning of the period	$ 134	$ 144	$ 139	$ 119
Net realized and unrealized losses (gains)/losses	(4)	9	(8)	35
Transfers out of Level 3 to Level 2	(1)	0	(2)	(1)
Balance as of the end of the period	$ 129	$ 153	$ 129	$ 153