Computation of Basic and Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2019		Sep. 30, 2019		Jun. 30, 2019		Mar. 31, 2019		Dec. 31, 2018		Sep. 30, 2018		Jun. 30, 2018		Mar. 31, 2018		Dec. 31, 2019		Dec. 31, 2018		Dec. 31, 2017
Earnings Loss Per Share [Line Items]
Net income	$ 12,167		$ 9,700		$ 20,394		$ (7,906)		$ 3,301		$ 12,699		$ 4,828		$ 1,673		$ 34,355		$ 22,501		$ 18,195
Total weighted average common stock shares outstanding																	10,256		11,375		11,444
Employee stock options																	72		37		103
Adjusted weighted average shares																	10,328		11,412		11,547
Earnings per share:
Basic earnings per share	$ 1.19	[1]	$ 0.95	[1]	$ 1.99	[1]	$ (0.77)	[1]	$ 0.30	[1]	$ 1.11	[1]	$ 0.42	[1]	$ 0.15	[1]	$ 3.35	[1],[2]	$ 1.98	[1],[2]	$ 1.59	[2]
Diluted earnings per share	$ 1.18	[1]	$ 0.94	[1]	$ 1.98	[1]	$ (0.77)	[1]	$ 0.30	[1]	$ 1.10	[1]	$ 0.42	[1]	$ 0.15	[1]	$ 3.33	[1],[2]	$ 1.97	[1],[2]	$ 1.58	[2]
Class A
Earnings Loss Per Share [Line Items]
Net income																	$ 0		$ 0		$ 0
Total weighted average common stock shares outstanding																	8,221		9,340		9,409
Class B Convertible
Earnings Loss Per Share [Line Items]
Net income																	$ 0		$ 0		$ 0
Total weighted average common stock shares outstanding																	2,035		2,035		2,035

[1] The sum of the basic and diluted earnings (loss) per share for the four quarters may differ from annual earnings per share as the weighted average shares outstanding are computed independently for each of the quarters presented.

[2] Basic and fully diluted earnings per share for class A and class B common stock are the same.