Stockholders' Equity - Narrative (Details) - USD ($) $ in Millions					12 Months Ended
	Dec. 31, 2017	Nov. 30, 2015	Aug. 18, 2015	May 22, 2015	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Aug. 03, 2017
Equity, Class of Treasury Stock [Line Items]
Common stock repurchased and retired during the period (in shares)		1,000,000	2,300,000	2,000,000
Common stock repurchased and retired during the period					$ 534.0	$ 367.5	$ 241.3
Treasury stock acquired (in shares)	109,207
The Share Repurchase Program
Equity, Class of Treasury Stock [Line Items]
Common stock repurchased and retired during the period (in shares)					8,893,993
Common stock repurchased and retired during the period					$ 534.0
Additional amount authorized under repurchase program								$ 750.0
Amount remaining under repurchase program	$ 858.0				$ 858.0