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					12 Months Ended
	Feb. 29, 2020	Nov. 30, 2018	May 31, 2018	May 31, 2017	Feb. 29, 2016	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Common Stock Dividends
Dividends declared per common share (in dollars per share)						$ 2.24	$ 2.18	$ 2.10
Reconciliation of Outstanding Common Shares
Outstanding shares at beginning of period						276.6	279.5	289.0
Shares issued						2.6	2.6	2.6
Treasury stock acquired						(5.9)	(5.5)	(12.1)
Outstanding shares at end of period						273.3	276.6	279.5
Common stock share repurchase disclosures
Share repurchase program, maximum authorized amount (in dollars)	$ 900.0	$ 500.0	$ 300.0	$ 250.0	$ 400.0