Shareholders Equity - Schedule of Repurchase of Shares (Detail) - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)	500,989	612,034
Average price of shares repurchased (in dollars per share)	$ 59.83	$ 59.75
Total cost	$ 30.0	$ 36.6
10b5-1 Trading Plan Initiated December 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)	75,661
Average price of shares repurchased (in dollars per share)	$ 59.30
Total cost	$ 4.5
10b5-1 Trading Plan Initiated September 2018 [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)	7,492
Average price of shares repurchased (in dollars per share)	$ 59.86
Total cost	$ 0.5
Open Market Repurchase Period 1 [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
Open Market Repurchase Period 2 [Member]
Equity, Class of Treasury Stock [Line Items]
Number of shares repurchased (in shares)	103,250
Average price of shares repurchased (in dollars per share)	$ 62.07
Total cost	$ 6.4