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, 2015	$ 167,273				$ 6	$ 2	$ 1		$ 910,228	$ (738,849)	$ (4,115)
Balance, Beginning, Shares at Dec. 31, 2015					64,477,171	14,927,613	9,352,729
Issuance of common stock upon exercise of stock options or awards of restricted stock units	$ 781				$ 1				780
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares	925,000				1,409,085
Stock-based compensation expense	$ 5,035								5,035
Dividends paid	(11,176)								(11,176)
Change in fair value of interest rate swap agreements	1,138										1,138
Net income	20,405									20,405
Balance, Ending at Dec. 31, 2016	183,456				$ 7	$ 2	$ 1		904,867	(718,444)	(2,977)
Balance, Ending, Shares at Dec. 31, 2016					65,886,256	14,927,613	9,352,729
Adoption of ASU 2016-09	2,385								(36)	2,421
Issuance of common stock upon exercise of stock options or awards of restricted stock units	$ 708								708
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares	915,000				603,440
Tax payments related to shares withheld for share-based compensation plans	$ (2,980)								(2,980)
Tax payments related to shares withheld for share-based compensation plans, shares					537,886
Stock-based compensation expense	6,091								6,091
Repurchase of Class A common stock	(5,330)								(5,330)
Repurchase of Class A common stock, Shares					(958,257)			958,257
Retirement of treasury stock, Shares								(958,257)
Dividends paid	(14,670)								(14,670)
Change in fair value of interest rate swap agreements	1,530										1,530
OCI release due to termination of interest rate swap agreements	1,447										1,447
Foreign currency translation gain (loss)	(60)										(60)
Net income	175,698									175,698
Balance, Ending at Dec. 31, 2017	348,275				$ 7	$ 2	$ 1		888,650	(540,325)	(60)
Balance, Ending, Shares at Dec. 31, 2017		66,069,325	14,927,613	9,352,729	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)	$ (13,700)							(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	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	63,210,531	14,927,613	9,352,729