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		9 Months Ended
	Feb. 29, 2020	Nov. 30, 2018	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Common Stock Dividends
Dividends declared per common share (in dollars per share)			$ 0.56	$ 0.55	$ 1.68	$ 1.63
Reconciliation of Outstanding Common Shares
Outstanding shares at beginning of period			274.0	278.6	276.6	279.5
Shares issued			0.6	0.2	2.4	2.4
Treasury stock acquired			(0.1)	(0.8)	(4.5)	(3.9)
Outstanding shares at end of period			274.5	278.0	274.5	278.0
Common stock share repurchase disclosures
Share repurchase program, maximum authorized amount (in dollars)	$ 900.0	$ 500.0