Selected Quarterly Financial Data (Unaudited) (Detail) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 140,108		$ 129,896		$ 123,142		$ 119,015		$ 143,522		$ 129,536		$ 132,940		$ 123,871		$ 512,161		$ 529,869		$ 579,830
Gross profit	116,820		106,961		99,041		96,192		120,108		104,852		107,028		96,773		419,014		428,761		444,620
Net income	$ 31,124		$ 26,628		$ 18,884		$ 14,272		$ 39,111		$ 23,893		$ 22,467		$ 20,460		$ 90,908		$ 105,931		$ 5,035
Earnings per share:
Basic	$ 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	$ 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]

[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.