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, 2016		Sep. 30, 2016		Jun. 30, 2016		Mar. 31, 2016		Dec. 31, 2015		Sep. 30, 2015		Jun. 30, 2015		Mar. 31, 2015		Dec. 31, 2016		Dec. 31, 2015		Dec. 31, 2014
Earnings Loss Per Share [Line Items]
Net income	$ 31,124		$ 26,628		$ 18,884		$ 14,272		$ 39,111		$ 23,893		$ 22,467		$ 20,460		$ 90,908		$ 105,931		$ 5,035
Total weighted average common stock shares outstanding																	11,425		11,355		11,301
Employee stock options																	91		184		55
Adjusted weighted average shares																	11,516		11,539		11,356
Earnings per share:
Basic earnings per share	$ 2.72	[1]	$ 2.33	[1]	$ 1.65	[1]	$ 1.25	[1]	$ 3.44	[1]	$ 2.10	[1]	$ 1.98	[1]	$ 1.81	[1]	$ 7.96	[1],[2]	$ 9.33	[1],[2]	$ 0.45	[2]
Diluted earnings per share	$ 2.69	[1]	$ 2.31	[1]	$ 1.64	[1]	$ 1.24	[1]	$ 3.38	[1]	$ 2.06	[1]	$ 1.95	[1]	$ 1.79	[1]	$ 7.89	[1],[2]	$ 9.18	[1],[2]	$ 0.44	[2]
Class A
Earnings Loss Per Share [Line Items]
Net income																	$ 0		$ 0		$ 0
Total weighted average common stock shares outstanding																	9,390		9,320		9,246
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		2,055

[1] The sum of the basic and diluted earnings 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.