UNAUDITED CONSOLIDATED STATEMENTS SOTCKHOLDERS' EQUITY - USD ($) $ in Thousands	Total	Class A and B Common Stock [Member]	Additional Paid-in Capital [Member]	Purchase Price Of Excess Of Predecessor Basis, Net Of Tax [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income [Member]	Treasury Stock [Member]
Beginning Balance at Dec. 31, 2017	$ 769,872	$ 419	$ 173,011	$ (15,458)	$ 870,716	$ (194)	$ (258,622)
Beginning Balance (in shares) at Dec. 31, 2017		41,887,088					(7,777,722)
Stock tendered for payments of withholding taxes	(4,017)						$ (4,017)
Stock tendered for payments of withholding taxes (in shares)							(82,265)
Issuance of restricted stock awards, net of forfeitures			(11,379)				$ 11,379
Issuance of restricted stock awards, net of forfeitures (in shares)							352,364
Share-based compensation expense	6,982		6,982
Net income	38,218				38,218
Foreign currency translation adjustment	4					4
Ending Balance at Jun. 30, 2018	811,059	$ 419	168,614	(15,458)	908,934	(190)	$ (251,260)
Ending Balance (in shares) at Jun. 30, 2018		41,887,088					(7,507,623)
Beginning Balance at Mar. 31, 2018	785,674	$ 419	165,078	(15,458)	886,883	(165)	$ (251,083)
Beginning Balance (in shares) at Mar. 31, 2018		41,887,088					(7,510,187)
Stock tendered for payments of withholding taxes	(123)						$ (123)
Stock tendered for payments of withholding taxes (in shares)							(2,775)
Issuance of restricted stock awards, net of forfeitures			54				$ (54)
Issuance of restricted stock awards, net of forfeitures (in shares)							5,339
Share-based compensation expense	3,482		3,482
Net income	22,051				22,051
Foreign currency translation adjustment	(25)					(25)
Ending Balance at Jun. 30, 2018	811,059	$ 419	168,614	(15,458)	908,934	(190)	$ (251,260)
Ending Balance (in shares) at Jun. 30, 2018		41,887,088					(7,507,623)
Beginning Balance at Dec. 31, 2018	980,834	$ 419	172,220	(15,458)	1,072,456	(182)	$ (248,621)
Beginning Balance (in shares) at Dec. 31, 2018		41,887,088					(7,431,083)
Purchase of treasury shares	(7,282)						$ (7,282)
Purchase of treasury shares (in shares)							(181,811)
Stock tendered for payments of withholding taxes	(2,746)						$ (2,746)
Stock tendered for payments of withholding taxes (in shares)							(72,713)
Issuance of restricted stock awards, net of forfeitures			(10,288)				$ 10,288
Issuance of restricted stock awards, net of forfeitures (in shares)							316,555
Share-based compensation expense	8,687		8,687
Net income	53,111				53,111
Adoption of ASU 2016-02 | ASU No. 2016-02 [Member]	(26)				(26)
Foreign currency translation adjustment	4					4
Ending Balance at Jun. 30, 2019	1,032,582	$ 419	170,619	(15,458)	1,125,541	(178)	$ (248,361)
Ending Balance (in shares) at Jun. 30, 2019		41,887,088					(7,369,052)
Beginning Balance at Mar. 31, 2019	1,007,064	$ 419	163,340	(15,458)	1,096,324	(175)	$ (237,386)
Beginning Balance (in shares) at Mar. 31, 2019		41,887,088					(7,114,310)
Purchase of treasury shares	(7,282)						$ (7,282)
Purchase of treasury shares (in shares)							(181,811)
Stock tendered for payments of withholding taxes	(168)						$ (168)
Stock tendered for payments of withholding taxes (in shares)							(3,805)
Issuance of restricted stock awards, net of forfeitures			3,525				$ (3,525)
Issuance of restricted stock awards, net of forfeitures (in shares)							(69,126)
Share-based compensation expense	3,754		3,754
Net income	29,217				29,217
Foreign currency translation adjustment	(3)					(3)
Ending Balance at Jun. 30, 2019	$ 1,032,582	$ 419	$ 170,619	$ (15,458)	$ 1,125,541	$ (178)	$ (248,361)
Ending Balance (in shares) at Jun. 30, 2019		41,887,088					(7,369,052)