Computation of Earnings Per Share - Summary of Calculation of Basic Earnings Per Common Share and Diluted Earnings Per Common Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Numerator:
Net income	$ 40,236	$ 41,539	$ 28,454	$ 28,111	$ 21,419	$ 32,968	$ 37,073	$ 22,259	$ 138,340	$ 113,719	$ 104,981
Denominator:
Denominator for basic earnings per share–weighted average common shares									26,140	25,952	25,762
Effect of dilutive stock options and restricted stock									108	126	160
Effect of other common stock equivalents									344	357	369
Denominator for diluted earnings per share–adjusted weighted average common shares									26,592	26,435	26,291
Basic Earnings Per Share	$ 1.54	$ 1.59	$ 1.09	$ 1.08	$ 0.82	$ 1.27	$ 1.43	$ 0.86	$ 5.29	$ 4.38	$ 4.08
Diluted Earnings Per Share	$ 1.51	$ 1.56	$ 1.07	$ 1.06	$ 0.81	$ 1.25	$ 1.40	$ 0.85	$ 5.20	$ 4.30	$ 3.99