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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Numerator:
Net income	$ 25,171	$ 29,471	$ 42,820	$ 37,110	$ 18,299	$ 20,098	$ 28,782	$ 27,949	$ 134,572	$ 95,128	$ 67,847
Denominator:
Basic weighted average shares outstanding	58,603,537	58,741,973	58,760,753	58,855,156	58,825,696	58,808,098	58,778,916	58,726,420	58,739,670	58,785,118	58,789,118
Effect of dilutive securities:
Diluted weighted average shares outstanding	58,936,795	59,084,117	59,051,413	59,199,622	59,222,258	59,156,432	59,100,836	59,122,812	59,067,302	59,150,920	59,225,338
Basic earnings per share	$ 0.43	$ 0.50	$ 0.73	$ 0.63	$ 0.31	$ 0.34	$ 0.49	$ 0.48	$ 2.29	$ 1.62	$ 1.16
Diluted earnings per share	$ 0.43	$ 0.50	$ 0.73	$ 0.63	$ 0.31	$ 0.34	$ 0.49	$ 0.48	$ 2.28	$ 1.61	$ 1.15
Stock Appreciation Rights [Member]
Effect of dilutive securities:
Dilutive securities									176,700	198,642	250,238
Restricted Stock [Member]
Effect of dilutive securities:
Dilutive securities									150,932	167,160	185,982