Share Repurchases - Additional Information (Details) - USD ($) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Nov. 30, 2017	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017	Nov. 01, 2017
Accelerated Share Repurchases [Line Items]
Stock repurchase program, authorized amount							$ 500.0
Payment for share repurchase	$ 100.0		$ 200.0	$ 200.0	$ 300.0
Accelerated share repurchase contract amount						$ 200.0
Stock purchases, shares		0	4,100,000
Evergreen Program
Accelerated Share Repurchases [Line Items]
Payment for share repurchase			$ 110.0
Repurchase Program
Accelerated Share Repurchases [Line Items]
Payment for share repurchase			$ 90.0
Remaining amount for share repurchase program		$ 310.0		$ 310.0