Summary of Accounting Policies - Redeemable Non Controlling Interest (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2019	Mar. 31, 2018
Redeemable Noncontrolling Interest [Roll Forward]
Balance at beginning of period	$ 138	$ 42
Net income (loss) attributable to redeemable noncontrolling interests	5	7
Other comprehensive income (loss)	2	1
Acquisition and other	16	0
Purchase accounting measurement period adjustment	(8)	0
Balance at end of period	$ 153	$ 50