Summary of Stock Option Activity (Detail) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017 USD ($) $ / shares shares
Number of options and SAR units
Number of shares outstanding, Beginning balance | shares	1,455,908	[1]
Number of shares, Options/SAR units granted | shares	0	[1]
Number of shares, Options/SAR units exercised | shares	(711,208)	[1]
Number of shares, Options/SAR units forfeited or expired | shares	(15,683)	[1]
Number of shares outstanding, Ending balance | shares	729,017	[1],[2]
Number of shares exercisable, Ending balance | shares	513,464	[1],[3]
Weighted-average exercise price
Weighted-average exercise price, Beginning balance | $ / shares	$ 19.76
Weighted average exercise price, Granted | $ / shares	0
Weighted average exercise price, Exercised | $ / shares	19.80
Weighted average exercise price, Forfeited or expired | $ / shares	17.51
Weighted average exercise price, Ending balance | $ / shares	19.77	[2]
Weighted average exercise price, exercisable | $ / shares	$ 19.07	[3]
Weighted average remaining contractual term
Weighted average remaining contractual life, Outstanding at end of period	5 years 2 months 12 days	[2]
Weighted average remaining contractual life, Exercisable at end of period	4 years 8 months 12 days	[3]
Aggregate intrinsic value
Aggregate intrinsic value, Outstanding | $	$ 19,229	[2]
Aggregate intrinsic value, Exercisable | $	$ 13,906	[3]

[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 662,075 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 465,223 shares of the Company's common stock issuable upon exercise.