Stock-Based Compensation - Summary of Stock Appreciation Rights (Detail) $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares
Time-Based Stock Appreciation Rights [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Outstanding, Beginning Balance	2,564,394
Number of Shares, Granted	0
Number of Shares, Exercised	(238,955)
Number of Shares, Forfeited or expired	(20,000)
Number of Shares, Outstanding, Ending Balance	2,305,439
Number of Shares, Exercisable	2,245,439
Number of Shares, Vested and expected to vest	2,305,439
Weighted-Average Exercise Price, Outstanding, Beginning Balance | $ / shares	$ 5.38
Weighted-Average Exercise Price, Exercised | $ / shares	4.77
Weighted-Average Exercise Price, Forfeited or expired | $ / shares	8.58
Weighted-Average Exercise Price, Outstanding, Ending Balance | $ / shares	5.33
Weighted-Average Exercise Price, Exercisable | $ / shares	5.38
Weighted-Average Exercise Price, Vested and expected to vest | $ / shares	$ 5.33
Weighted-Average Remaining Contractual Term, Outstanding	4 years
Weighted-Average Remaining Contractual Term, Exercisable	3 years 10 months 20 days
Weighted-Average Remaining Contractual Term, Vested and expected to vest	4 years
Aggregate Intrinsic Value, Outstanding, Ending balance | $	$ 11,626
Aggregate Intrinsic Value, Exercisable | $	12,111
Aggregate Intrinsic Value, Vested and expected to vest | $	$ 11,626
Performance-Based Stock Appreciation Rights [Member]
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Number of Shares, Outstanding, Beginning Balance	504,473
Number of Shares, Granted	0
Number of Shares, Exercised	(144,940)
Number of Shares, Outstanding, Ending Balance	359,533
Number of Shares, Exercisable	359,533
Number of Shares, Vested and expected to vest	359,533
Weighted-Average Exercise Price, Outstanding, Beginning Balance | $ / shares	$ 5.80
Weighted-Average Exercise Price, Exercised | $ / shares	2.84
Weighted-Average Exercise Price, Outstanding, Ending Balance | $ / shares	5.96
Weighted-Average Exercise Price, Exercisable | $ / shares	5.96
Weighted-Average Exercise Price, Vested and expected to vest | $ / shares	$ 5.96
Weighted-Average Remaining Contractual Term, Outstanding	3 years 3 months
Weighted-Average Remaining Contractual Term, Exercisable	3 years 3 months
Weighted-Average Remaining Contractual Term, Vested and expected to vest	3 years 3 months
Aggregate Intrinsic Value, Outstanding, Ending balance | $	$ 2,538
Aggregate Intrinsic Value, Exercisable | $	2,538
Aggregate Intrinsic Value, Vested and expected to vest | $	$ 2,538