Consolidated Statements of Stockholders' Equity - USD ($) $ in Thousands	Total	Class A common stock	Class B common stock	Class U common stock	Common Stock Class A common stock	Common Stock Class B common stock	Common Stock Class U common stock	Treasury Stock	Additional Paid-in Capital	Accumulated Deficit	Accumulated Other Comprehensive Income (Loss)
Balance, Beginning at Dec. 31, 2019	$ 288,172				$ 6	$ 2	$ 1		$ 836,170	$ (547,876)	$ (131)
Balance, Beginning, Shares at Dec. 31, 2019					60,074,698	14,927,613	9,352,729
Stock-based compensation expense	789								789
Repurchase of Class A common stock	(525)								(525)
Repurchase of Class A common stock					(259,500)			259,500
Retirement of treasury stock								(259,500)
Dividends paid	(4,218)								(4,218)
Change in fair value of marketable securities	(223)										(223)
Foreign currency translation gain (loss)	(166)										(166)
Net income (loss) attributable to common stockholders	(35,592)									(35,592)
Balance, Ending at Mar. 31, 2020	248,237				$ 6	$ 2	$ 1		832,216	(583,468)	(520)
Balance, Ending, Shares at Mar. 31, 2020					59,815,198	14,927,613	9,352,729
Balance, Beginning at Dec. 31, 2019	288,172				$ 6	$ 2	$ 1		836,170	(547,876)	(131)
Balance, Beginning, Shares at Dec. 31, 2019					60,074,698	14,927,613	9,352,729
Foreign currency translation gain (loss)	19
Net income (loss) attributable to common stockholders	(24,238)
Balance, Ending at Sep. 30, 2020	257,540				$ 6	$ 2	$ 1		829,610	(572,114)	35
Balance, Ending, Shares at Sep. 30, 2020					59,905,386	14,927,613	9,352,729
Balance, Beginning at Mar. 31, 2020	248,237				$ 6	$ 2	$ 1		832,216	(583,468)	(520)
Balance, Beginning, Shares at Mar. 31, 2020					59,815,198	14,927,613	9,352,729
Issuance of common stock upon exercise of stock options or awards of restricted stock units	(15)								(15)
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares					90,188
Stock-based compensation expense	803								803
Dividends paid	(2,104)								(2,104)
Change in fair value of marketable securities	534										534
Foreign currency translation gain (loss)	467										467
Net income (loss) attributable to common stockholders	2,338									2,338
Balance, Ending at Jun. 30, 2020	250,260				$ 6	$ 2	$ 1		830,900	(581,130)	481
Balance, Ending, Shares at Jun. 30, 2020					59,905,386	14,927,613	9,352,729
Stock-based compensation expense	816								816
Dividends paid	(2,106)								(2,106)
Change in fair value of marketable securities	(164)										(164)
Foreign currency translation gain (loss)	(282)										(282)
Net income (loss) attributable to common stockholders	9,016									9,016
Balance, Ending at Sep. 30, 2020	257,540				$ 6	$ 2	$ 1		829,610	(572,114)	35
Balance, Ending, Shares at Sep. 30, 2020					59,905,386	14,927,613	9,352,729
Balance, Beginning at Dec. 31, 2020	275,980				$ 6	$ 2	$ 1		828,813	(551,786)	(1,056)
Balance, Beginning, Shares at Dec. 31, 2020		60,759,405	14,927,613	9,352,729	60,759,405	14,927,613	9,352,729
Issuance of common stock upon exercise of stock options or awards of restricted stock units	(9)								(9)
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares					6,045
Stock-based compensation expense	1,071								1,071
Dividends paid	(2,126)								(2,126)
Change in fair value of marketable securities	(69)										(69)
Foreign currency translation gain (loss)	407										407
Net income (loss) attributable to common stockholders	5,429									5,429
Balance, Ending at Mar. 31, 2021	280,683				$ 6	$ 2	$ 1		827,749	(546,357)	(718)
Balance, Ending, Shares at Mar. 31, 2021					60,765,450	14,927,613	9,352,729
Balance, Beginning at Dec. 31, 2020	275,980				$ 6	$ 2	$ 1		828,813	(551,786)	(1,056)
Balance, Beginning, Shares at Dec. 31, 2020		60,759,405	14,927,613	9,352,729	60,759,405	14,927,613	9,352,729
Repurchase of Class A common stock		0
Foreign currency translation gain (loss)	366
Net income (loss) attributable to common stockholders	25,424
Balance, Ending at Sep. 30, 2021	253,270				$ 6	$ 2	$ 1		780,426	(526,362)	(803)
Balance, Ending, Shares at Sep. 30, 2021		61,746,028	14,327,613	9,352,729	61,746,028	14,327,613	9,352,729
Balance, Beginning at Mar. 31, 2021	280,683				$ 6	$ 2	$ 1		827,749	(546,357)	(718)
Balance, Beginning, Shares at Mar. 31, 2021					60,765,450	14,927,613	9,352,729
Issuance of common stock upon exercise of stock options or awards of restricted stock units	(277)								(277)
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares					304,194
Stock-based compensation expense	1,135								1,135
Class B common stock exchanged for Class A common stock, shares					120,000	(120,000)
Dividends paid	(2,133)								(2,133)
Change in fair value of marketable securities	(39)										(39)
Foreign currency translation gain (loss)	(271)										(271)
Net income (loss) attributable to common stockholders	7,864									7,864
Balance, Ending at Jun. 30, 2021	286,962				$ 6	$ 2	$ 1		826,474	(538,493)	(1,028)
Balance, Ending, Shares at Jun. 30, 2021					61,189,644	14,807,613	9,352,729
Issuance of common stock upon exercise of stock options or awards of restricted stock units	171								171
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares					76,384
Stock-based compensation expense	1,094								1,094
Class B common stock exchanged for Class A common stock, shares					480,000	(480,000)
Repurchase of Class A common stock		0
Dividends paid	(2,136)								(2,136)
Change in fair value of marketable securities	(5)										(5)
Foreign currency translation gain (loss)	230										230
Acquisition of redeemable noncontrolling interest	(45,177)								(45,177)
Net income (loss) attributable to common stockholders	12,131									12,131
Balance, Ending at Sep. 30, 2021	$ 253,270				$ 6	$ 2	$ 1		$ 780,426	$ (526,362)	$ (803)
Balance, Ending, Shares at Sep. 30, 2021		61,746,028	14,327,613	9,352,729	61,746,028	14,327,613	9,352,729