Selected Quarterly Financial Data (Unaudited) (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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 131,917		$ 122,152		$ 120,602		$ 122,967		$ 136,381		$ 126,010		$ 119,220		$ 122,232		$ 497,638		$ 503,843		$ 513,589
Gross profit	106,032		98,763		95,562		97,782		111,628		101,621		94,845		99,100		398,139		407,194		420,442
Net income (loss)	$ 3,301		$ 12,699		$ 4,828		$ 1,673		$ (25,499)		$ 18,184		$ 9,953		$ 15,557		$ 22,501		$ 18,195		$ 92,239
Earnings (loss) per share:
Basic	$ 0.30	[1]	$ 1.11	[1]	$ 0.42	[1]	$ 0.15	[1]	$ (2.23)	[1]	$ 1.59	[1]	$ 0.87	[1]	$ 1.36	[1]	$ 1.98	[1],[2]	$ 1.59	[1],[2]	$ 8.07	[2]
Diluted	$ 0.30	[1]	$ 1.10	[1]	$ 0.42	[1]	$ 0.15	[1]	$ (2.23)	[1]	$ 1.58	[1]	$ 0.86	[1]	$ 1.34	[1]	$ 1.97	[1],[2]	$ 1.58	[1],[2]	$ 8.01	[2]

[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.