Share Repurchases - Additional Information (Details) - USD ($) shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Nov. 01, 2017
Accelerated Share Repurchases [Line Items]
Stock repurchase program, authorized amount				$ 500.0
Stock purchases, shares	8.2
Payment for share repurchase	$ 400.0	$ 400.0	$ 363.1
Evergreen Program
Accelerated Share Repurchases [Line Items]
Payment for share repurchase	125.0
2016 Repurchase Program
Accelerated Share Repurchases [Line Items]
Payment for share repurchase	175.0
Repurchase Program
Accelerated Share Repurchases [Line Items]
Payment for share repurchase	100.0
Remaining amount for share repurchase program	$ 400.0