Stockholders' Equity - Reconciliation of Outstanding Shares (Details) - Common stock - USD ($) shares in Millions, $ in Millions	1 Months Ended		3 Months Ended
	Feb. 29, 2016	Oct. 31, 2015	Mar. 31, 2017	Mar. 31, 2016
Reconciliation of Outstanding Shares
Outstanding shares at beginning of period			287.7	291.4
Shares issued			2.7	1.9
Treasury stock acquired			(2.3)	(2.9)
Outstanding shares at end of period			288.1	290.4
Common stock share repurchase disclosures
Share repurchase program, maximum authorized amount (in dollars)	$ 400.0	$ 150.0