Summary of Significant Accounting Policies - Redeemable Noncontrolling Interest (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Noncontrolling Interest [Line Items]
Balance as of beginning of period			$ 46
Contributions from redeemable noncontrolling interest			42
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable noncontrolling interest	$ (17)	$ (16)	(56)	$ (68)
Balance as of end of period	$ 51		51
Redeemable noncontrolling interest [Member]
Noncontrolling Interest [Line Items]
Less: Comprehensive (loss)/income attributable to noncontrolling interest and redeemable noncontrolling interest			$ (37)