Summary of Stock Appreciation Rights Activity (Detail) - Stock Appreciation Rights [Member] - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Shares, Grants outstanding at beginning of the period	100,000	170,000	170,000
Shares, Exercised		(67,812)
Shares, Forfeited		(2,188)
Shares, Grants outstanding at end of the period	100,000	100,000	170,000
Shares, Grants exercisable at end of the period	100,000	100,000	141,249
Shares, Grants vested and expected to vest at end of the period	100,000	100,000	167,813
Weighted Average Exercise Price per Share, Grants outstanding at beginning of the period	$ 23.85	$ 18.18	$ 18.18
Weighted Average Exercise Price per Share, Exercised		10.08
Weighted Average Grant Date Fair Value per Share, Forfeited		10.08
Weighted Average Exercise Price per Share, Grants outstanding at end of the period	23.85	23.85	18.18
Weighted Average Exercise Price per Share, Grants exercisable at end of the period	23.85	23.85	17.59
Weighted Average Exercise Price per Share, Grants vested and expected to vest at end of the period	$ 23.85	$ 23.85	$ 18.29
Stock Appreciation Rights, Aggregate Intrinsic Value
SARs outstanding, exercisable and vested at December 31, 2017	$ 3,179
Stock Appreciation Rights, Weighted Average Remaining Contractual Life
SARs outstanding, exercisable and vested at December 31, 2017	4 years 10 months 24 days