Shareholders Equity - Schedule of Repurchase of Shares (Detail) - 10b5-1 Trading Plan Initiated In 06/14/2017 [Member] $ / shares in Units, $ in Millions	6 Months Ended
	Jun. 30, 2016 USD ($) $ / shares shares
Equity Class Of Treasury Stock [Line Items]
Date Trading Plan Initiated	Jun. 14, 2017
Trading Plan Purchase Period Start Date	Jun. 20, 2017
Common shares repurchased on open market | shares	46,500
Average Price of Shares Repurchased | $ / shares	$ 59.54
Total Cost (in millions) | $	$ 2.8
Repurchase Authorization Year	2016
Trading Plan Purchase Period End Date	Jun. 28, 2017