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, 2017	$ 348,275				$ 7	$ 2	$ 1		$ 888,650	$ (540,325)	$ (60)
Balance, Beginning, Shares at Dec. 31, 2017					66,069,325	14,927,613	9,352,729
Issuance of common stock upon exercise of stock options or awards of restricted stock units	$ 249								249
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares	177,000				100,000
Tax payments related to shares withheld for share-based compensation plans	$ (794)				$ (1)				(793)
Tax payments related to shares withheld for share-based compensation plans, shares					586,306
Stock-based compensation expense	5,787								5,787
Repurchase of Class A common stock	(13,812)	$ (13,800)							(13,812)
Repurchase of Class A common stock, Shares		(3,500,000)			(3,545,100)			3,545,100
Retirement of treasury stock, Shares								(3,545,100)
Dividends paid	(17,782)								(17,782)
Change in fair value of marketable securities	(1,000)										(1,000)
Foreign currency translation gain (loss)	(352)										(352)
Net income (loss) attributable to common stockholders	12,161									12,161
Balance, Ending at Dec. 31, 2018	$ 332,732				$ 6	$ 2	$ 1		862,299	(528,164)	(1,412)
Balance, Ending, Shares at Dec. 31, 2018					63,210,531	14,927,613	9,352,729
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares	49,000
Tax payments related to shares withheld for share-based compensation plans	$ (979)								(979)
Tax payments related to shares withheld for share-based compensation plans, shares					673,732
Stock-based compensation expense	4,377								4,377
Repurchase of Class A common stock	(12,565)	$ (12,600)							(12,565)
Repurchase of Class A common stock, Shares		(3,800,000)			(3,809,565)			3,809,565
Retirement of treasury stock, Shares								(3,809,565)
Dividends paid	(16,962)								(16,962)
Change in fair value of marketable securities	1,428										1,428
Foreign currency translation gain (loss)	(147)										(147)
Net income (loss) attributable to common stockholders	(19,712)									(19,712)
Balance, Ending at Dec. 31, 2019	$ 288,172				$ 6	$ 2	$ 1		836,170	(547,876)	(131)
Balance, Ending, Shares at Dec. 31, 2019		60,074,698	14,927,613	9,352,729	60,074,698	14,927,613	9,352,729
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares	10,000
Tax payments related to shares withheld for share-based compensation plans	$ (1,426)								(1,426)
Tax payments related to shares withheld for share-based compensation plans, shares					944,207
Stock-based compensation expense	5,125								5,125
Repurchase of Class A common stock	(525)	$ (500)							(525)
Repurchase of Class A common stock, Shares		(300,000)			(259,500)			259,500
Retirement of treasury stock, Shares								(259,500)
Dividends paid	(10,531)								(10,531)
Change in fair value of marketable securities	(3)										(3)
Foreign currency translation gain (loss)	(922)										(922)
Net income (loss) attributable to common stockholders	(3,910)									(3,910)
Balance, Ending at Dec. 31, 2020	$ 275,980				$ 6	$ 2	$ 1		$ 828,813	$ (551,786)	$ (1,056)
Balance, Ending, Shares at Dec. 31, 2020		60,759,405	14,927,613	9,352,729	60,759,405	14,927,613	9,352,729