Share Repurchases - Additional Information (Details) - USD ($) shares in Millions, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Nov. 01, 2017
Accelerated Share Repurchases [Line Items]
Stock repurchase program, authorized amount					$ 500.0
Accelerated share repurchase contract amount			$ 200.0
Stock purchases, shares	4.1
Payment for share repurchase	$ 200.0	$ 200.0	$ 400.0	$ 400.0
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