Fair Value of Financial Instruments Unobservable Input Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance as of the beginning of the period	$ 129	$ 153	$ 139	$ 119
Net realized and unrealized losses (gains)/losses	(2)	(1)	(10)	34
Transfers out of Level 3 to Level 2	(2)	1	(4)	0
Balance as of the end of the period	$ 125	$ 153	$ 125	$ 153