Shareholders' Equity - Common Stock (Details) - USD ($)	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2019	Jun. 30, 2018
2018 Program
Equity, Class of Treasury Stock [Line Items]
Common stock repurchased	$ 0	$ 300,000,000	$ 300,000,000	$ 600,000,000
Amount of common stock authorized for repurchase					$ 1,200,000,000
2019 Program
Equity, Class of Treasury Stock [Line Items]
Common stock repurchased	$ 500,000,000			$ 500,000,000
Amount of common stock authorized for repurchase		$ 2,000,000,000.0