Shareholders Equity - Schedule of Repurchase of Shares (Detail) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)	344,533	46,500
Average price of shares repurchased (in dollars per share)	$ 59.20	$ 59.54
Total cost	$ 20.4	$ 2.8
10b5-1 Trading Plan Initiated In 12/15/2017 Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)	225,281
Average price of shares repurchased (in dollars per share)	$ 59.31
Total cost	$ 13.3
10b5-1 Trading Plan Initiated 6/15/2018 [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)	29,947
Average price of shares repurchased (in dollars per share)	$ 58.86
Total cost	$ 1.8
Open Market Repurchase Authorization 2016 [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)	89,305
Average price of shares repurchased (in dollars per share)	$ 59.03
Total cost	$ 5.3