Stockholders' Equity - Common Stock Dividends and Reconciliation of Outstanding Common Shares (Details) - Common stock - USD ($) $ / shares in Units, shares in Millions, $ in Millions	1 Months Ended				3 Months Ended			6 Months Ended
	Nov. 30, 2018	May 31, 2018	May 31, 2017	Feb. 29, 2016	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Common Stock Dividends
Dividends declared per common share (in dollars per share)					$ 0.54		$ 0.52	$ 1.08	$ 1.03
Reconciliation of Outstanding Common Shares
Outstanding shares at beginning of period					278.2	279.5	287.7	279.5	289.0
Shares issued					0.4		0.3	2.2	2.2
Treasury stock acquired							(3.4)	(3.1)	(6.6)
Outstanding shares at end of period					278.6	278.2	284.6	278.6	284.6
Common stock share repurchase disclosures
Share repurchase program, maximum authorized amount (in dollars)	$ 500.0	$ 300.0	$ 250.0	$ 400.0
Remaining share repurchase authorization suspended						$ 295.3