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, 2014	$ 145,558				$ 6	$ 2	$ 1		$ 912,161	$ (764,474)	$ (2,138)
Balance, Beginning, Shares at Dec. 31, 2014					58,893,970	18,930,035	9,352,729
Issuance of common stock upon exercise of stock options or awards of restricted stock units	$ 2,177								2,177
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares	966,000				1,580,779
Stock-based compensation expense	$ 5,240								5,240
Class B common stock exchanged for Class A common stock					4,002,422	(4,002,422)
Dividends paid	(9,350)								(9,350)
Change in fair value of interest rate swap agreements	(1,977)										(1,977)
Net income	25,625									25,625
Balance, Ending at Dec. 31, 2015	167,273				$ 6	$ 2	$ 1		910,228	(738,849)	(4,115)
Balance, Ending, 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	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				$ 0				708
Issuance of common stock upon exercise of stock options or awards of restricted stock units, Shares	915,000				603,440
Taxes related to shares withheld for share-based compensation plans	$ (2,980)								(2,980)
Taxes 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)	$ (5,300)			$ 0				(5,330)
Repurchase of Class A common stock, Shares		(1,000,000)			(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	176,293									176,293
Balance, Ending at Dec. 31, 2017	$ 348,870				$ 7	$ 2	$ 1		$ 888,650	$ (539,730)	$ (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