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, Shares					17,356
Tax payments related to shares withheld for share-based compensation plans	(43)								(43)
Stock-based compensation expense	1,249								1,249
Repurchase of Class A common stock	(2,402)								(2,402)
Repurchase of Class A common stock, Shares					(500,587)			500,587
Retirement of treasury stock	0
Retirement of treasury stock, Shares								(500,587)
Dividends paid	(4,518)								(4,518)
Change in fair value of marketable securities	(1,239)										(1,239)
Foreign currency translation gain (loss)	290										290
Net income	(1,808)									(1,808)
Balance, Ending at Mar. 31, 2018	339,804				$ 7	$ 2	$ 1		882,936	(542,133)	(1,009)
Balance, Ending, Shares at Mar. 31, 2018					65,586,094	14,927,613	9,352,729
Balance, Beginning at Dec. 31, 2018	332,732				$ 6	$ 2	$ 1		862,299	(528,164)	(1,412)
Balance, Beginning, Shares at Dec. 31, 2018		63,210,531	14,927,613	9,352,729	63,210,531	14,927,613	9,352,729
Tax payments related to shares withheld for share-based compensation plans	(42)								(42)
Tax payments related to shares withheld for share-based compensation plans, shares					25,059
Stock-based compensation expense	800								800
Repurchase of Class A common stock	(7,706)	$ (7,700)							(7,706)
Repurchase of Class A common stock, Shares		(2,100,000)			(2,098,443)			2,098,443
Retirement of treasury stock	0
Retirement of treasury stock, Shares								(2,098,443)
Dividends paid	(4,271)								(4,271)
Change in fair value of marketable securities	763										763
Foreign currency translation gain (loss)	30										30
Net income	1,424									1,424
Balance, Ending at Mar. 31, 2019	$ 323,730				$ 6	$ 2	$ 1		$ 851,080	$ (526,740)	$ (619)
Balance, Ending, Shares at Mar. 31, 2019		61,137,147	14,927,613	9,352,729	61,137,147	14,927,613	9,352,729