Stockholders' Equity - Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Numerator:
Net income	$ 18,299	$ 20,098	$ 28,782	$ 27,949	$ 12,629	$ 7,787	$ 23,725	$ 23,706	$ 95,128	$ 67,847	$ 48,098
Denominator:
Basic weighted average shares outstanding	29,412,848	29,404,049	29,389,458	29,363,210	29,318,915	29,295,284	29,264,362	29,697,722	29,392,559	29,394,559	31,350,542
Effect of dilutive securities:
Diluted weighted average shares outstanding	29,611,129	29,578,216	29,550,418	29,561,406	29,543,842	29,516,718	29,477,870	29,910,292	29,575,460	29,612,669	31,682,509
Basic earnings per share	$ 0.62	$ 0.68	$ 0.98	$ 0.95	$ 0.43	$ 0.27	$ 0.81	$ 0.80	$ 3.24	$ 2.31	$ 1.53
Diluted earnings per share	$ 0.62	$ 0.68	$ 0.97	$ 0.95	$ 0.43	$ 0.26	$ 0.80	$ 0.79	$ 3.22	$ 2.29	$ 1.52
Stock Appreciation Rights [Member]
Effect of dilutive securities:
Dilutive securities									99,321	125,119	197,299
Restricted Stock [Member]
Effect of dilutive securities:
Dilutive securities									83,580	92,991	134,668