Quarterly Financial Summary (Unaudited) - Schedule of Quarterly Financial Summary (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended																12 Months Ended
	Dec. 31, 2020	[1]	Sep. 30, 2020	[1]	Jun. 30, 2020	[1]	Mar. 31, 2020	[1]	Dec. 31, 2019		Sep. 30, 2019	[2]	Jun. 30, 2019	[3]	Mar. 31, 2019		Dec. 31, 2020		Dec. 31, 2019		Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Revenue	$ 171,159		$ 169,126		$ 176,035		$ 170,994		$ 158,451		$ 154,959		$ 156,441		$ 157,853		$ 687,314		$ 627,704		$ 581,296
Operating income	36,189		32,074		37,368		37,303		32,013		24,294		184		29,971		142,934		86,462		71,298
NET INCOME	$ 27,051		$ 39,940		$ 28,167		$ 29,106		$ 27,516		$ 18,234		$ (559)		$ 24,791		$ 124,264		$ 69,982		$ 55,181
Net income (loss) per share Basic	$ 0.39	[4]	$ 0.58	[4]	$ 0.41	[4]	$ 0.42	[4]	$ 0.39	[4]	$ 0.26	[4]	$ (0.01)	[4]	$ 0.35	[4]	$ 1.79	[4]	$ 1.00	[4]	$ 0.79
Net income (loss) per share Diluted	$ 0.38	[4]	$ 0.56	[4]	$ 0.40	[4]	$ 0.41	[4]	$ 0.38	[4]	$ 0.25	[4]	$ (0.01)	[4]	$ 0.35	[4]	$ 1.74	[4]	$ 0.97	[4]	$ 0.77

[1] 2020 results include the Trident acquisition commencing on the March 2, 2020 date of acquisition.

[2] Third quarter of 2019 net income included a $7.1 million significant legal settlement for the Oregon arbitration matter.

[3] Second quarter of 2019 net income included a $30.0 million significant legal settlement for the FTC matter, of which $23.3 million was non-deductible for income tax purposes during the second quarter of 2019.

[4] Basic and diluted earnings per share are calculated independently for each of the quarters presented. Accordingly, the sum of the quarterly earnings per share amounts may not agree with the annual earnings per share amount for the corresponding year.