Shareholders Equity - Additional Information (Detail) - USD ($)						3 Months Ended		9 Months Ended
	Sep. 14, 2018	Aug. 06, 2018	Mar. 07, 2018	Sep. 15, 2017	Aug. 08, 2017	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Feb. 20, 2018	May 03, 2016
Class of Stock [Line Items]
Stock divided declared						15.00%		15.00%
Dividend declared per common share (in dollars per share)		$ 0.27			$ 0.23	$ 0.27	$ 0.23	$ 0.81	$ 0.70
Cash dividends paid - common shares, total	$ 9,300,000			$ 8,300,000				$ 28,000,000	$ 24,900,000
Common shares repurchased (in shares)								500,989	612,034
Common shares repurchased on open market, total cost								$ 30,000,000	$ 36,600,000
2016 Repurchase Authorization [Member]
Class of Stock [Line Items]
Total number of shares authorized for purchase											$ 150,000,000
Remaining number of shares available under repurchase authorization						$ 55,100,000		$ 55,100,000
Common shares repurchased (in shares)								500,989
Common shares repurchased on open market, total cost								$ 30,000,000
Stock Dividend [Member]
Class of Stock [Line Items]
Stock divided declared										15.00%
Additional stock issued as dividend (in shares)			4,397,520