Fair Value of Financial Instruments Unobservable Input Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance as of the beginning of the period	$ 70	$ 134	$ 88	$ 139
Net realized and unrealized (gains)/losses	3	(4)	(16)	(8)
Transfers out of Level 3 to Level 2	(1)	(1)	0	(2)
Balance as of the end of the period	$ 72	$ 129	$ 72	$ 129