Shareholders' Equity - Schedule of Repurchase of Shares (Detail) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Equity, Class of Treasury Stock [Line Items]
Common shares repurchased on open market (in shares)	530,882	756,252	847,111
Average Price of Shares Repurchased (in dollars per share)	$ 59.83	$ 59.76	$ 55.64
Total Cost (in millions)	$ 31.7	$ 45.2	$ 47.1
10b5-1 Trading Plan Initiated December 2017 [Member]
Equity, Class of Treasury Stock [Line Items]
Common shares repurchased on open market (in shares)	225,281
Average Price of Shares Repurchased (in dollars per share)	$ 59.31
Total Cost (in millions)	$ 13.3
10b5-1 Trading Plan Initiated June 2018 [Member]
Equity, Class of Treasury Stock [Line Items]
Common shares repurchased on open market (in shares)	75,661
Average Price of Shares Repurchased (in dollars per share)	$ 59.30
Total Cost (in millions)	$ 4.5
10b5-1 Trading Plan Initiated September 2018 [Member]
Equity, Class of Treasury Stock [Line Items]
Common shares repurchased on open market (in shares)	37,385
Average Price of Shares Repurchased (in dollars per share)	$ 59.75
Total Cost (in millions)	$ 2.2
Open Market Repurchase Period 1 [Member]
Equity, Class of Treasury Stock [Line Items]
Common shares repurchased on open market (in shares)	89,305
Average Price of Shares Repurchased (in dollars per share)	$ 59.03
Total Cost (in millions)	$ 5.3
Open Market Repurchase Period 2 [Member]
Equity, Class of Treasury Stock [Line Items]
Common shares repurchased on open market (in shares)	103,250
Average Price of Shares Repurchased (in dollars per share)	$ 62.07
Total Cost (in millions)	$ 6.4