Summary of Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands		6 Months Ended		12 Months Ended
		Jun. 30, 2018		Dec. 31, 2017
Number of options and SAR units
Number of shares outstanding, Beginning balance	[1]	729,017
Number of shares, Options/SAR units granted	[1]	0
Number of shares, Options/SAR units exercised	[1]	(12,911)
Number of shares, Options/SAR units forfeited or expired	[1]	(1,461)
Number of shares outstanding, Ending balance	[1]	714,645	[2]	729,017
Number of shares exercisable, Ending balance	[1],[3]	569,977
Weighted-average exercise price
Weighted-average exercise price, Beginning balance		$ 19.77
Weighted average exercise price, Granted		0
Weighted average exercise price, Exercised		15.90
Weighted average exercise price, Forfeited or expired		18.82
Weighted average exercise price, Ending balance		19.84	[2]	$ 19.77
Weighted average exercise price, exercisable	[3]	$ 18.74
Weighted average remaining contractual term
Weighted average remaining contractual life, Outstanding at beginning of period		4 years 9 months 18 days	[2]	5 years 2 months 12 days
Weighted average remaining contractual life, Exercisable at end of period	[3]	4 years 2 months 12 days
Aggregate intrinsic value
Aggregate intrinsic value, Outstanding, at beginning of period		$ 7,430	[2]	$ 19,229
Aggregate intrinsic value, Exercisable at ending of period	[3]	$ 6,552

[1] The SAR units are convertible for a maximum number of shares of the Company's common stock equal to 75% of the SAR units subject to the grant.

[2] Due to the ceiling imposed on the SAR grants, the outstanding amount equals a maximum of 651,296 shares of the Company's common stock issuable upon exercise.

[3] Due to the ceiling imposed on the SAR grants, the exercisable amount equals a maximum of 515,045 shares of the Company's common stock issuable upon exercise.