Summary of Accounting Policies - Redeemable Non Controlling Interest (Details) - USD ($) $ in Millions	6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018
Redeemable Noncontrolling Interest [Roll Forward]
Balance at beginning of period	$ 138	$ 42
Net income (loss) attributable to redeemable noncontrolling interests	15	16
Other comprehensive income (loss)	1	(1)
Acquisition and other	16	0
Purchase accounting measurement period adjustment	(8)	0
Dividends declared	(17)	(19)
Balance at end of period	$ 145	$ 38