Fair Value of Financial Instruments Unobservable Input Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items]
Balance as of the beginning of the period	$ 88	$ 139
Net realized and unrealized losses (gains)/losses	(19)	(4)
Transfers out of Level 3 to Level 2	1	(1)
Balance as of the end of the period	$ 70	$ 134