Selected Quarterly Financial Data (Unaudited) (Detail) - USD ($) $ / shares in Units, $ 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
Quarterly Financial Information Disclosure [Abstract]
Revenues	$ 131,319		$ 127,408		$ 110,584		$ 111,424		$ 133,531		$ 119,693		$ 117,737		$ 115,366		$ 480,735		$ 486,327		$ 497,638
Gross profit	110,534		105,672		86,595		86,879		108,895		95,878		92,387		89,193		389,680		386,353		398,139
Net income (loss)	$ 2,661		$ (14,229)		$ 3,387		$ 657		$ 12,167		$ 9,700		$ 20,394		$ (7,906)		$ (7,524)		$ 34,355		$ 22,501
Earnings (loss) per share:
Basic earnings per share	$ 0.28	[1]	$ (1.48)	[1]	$ 0.35	[1]	$ 0.07	[1]	$ 1.19	[1]	$ 0.95	[1]	$ 1.99	[1]	$ (0.77)	[1]	$ (0.78)	[1],[2]	$ 3.35	[1],[2]	$ 1.98	[2]
Diluted earnings per share	$ 0.27	[1]	$ (1.48)	[1]	$ 0.35	[1]	$ 0.07	[1]	$ 1.18	[1]	$ 0.94	[1]	$ 1.98	[1]	$ (0.77)	[1]	$ (0.78)	[1],[2]	$ 3.33	[1],[2]	$ 1.97	[2]

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

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