UNAUDITED CONSOLIDATED STATEMENTS OF STOCKHOLDERS' 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	(4,262)						$ (4,262)
Stock tendered for payments of withholding taxes (in shares)							(87,206)
Issuance of restricted stock awards, net of forfeitures			(7,059)				$ 7,059
Issuance of restricted stock awards, net of forfeitures (in shares)							252,207
Share-based compensation expense	9,490		9,490
Net income	152,829				152,829
Foreign currency translation adjustment	26					26
Ending Balance at Sep. 30, 2018	927,955	$ 419	175,442	(15,458)	1,023,545	(168)	$ (255,825)
Ending Balance (in shares) at Sep. 30, 2018		41,887,088					(7,612,721)
Beginning Balance at Jun. 30, 2018	811,060	$ 419	168,614	(15,458)	908,935	(190)	$ (251,260)
Beginning Balance (in shares) at Jun. 30, 2018		41,887,088					(7,507,623)
Stock tendered for payments of withholding taxes	(245)						$ (245)
Stock tendered for payments of withholding taxes (in shares)							(4,941)
Issuance of restricted stock awards, net of forfeitures			4,320				$ (4,320)
Issuance of restricted stock awards, net of forfeitures (in shares)							(100,157)
Share-based compensation expense	2,508		2,508
Net income	114,610				114,610
Foreign currency translation adjustment	22					22
Ending Balance at Sep. 30, 2018	927,955	$ 419	175,442	(15,458)	1,023,545	(168)	$ (255,825)
Ending Balance (in shares) at Sep. 30, 2018		41,887,088					(7,612,721)
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	(24,998)						$ (24,998)
Purchase of treasury shares (in shares)							(626,320)
Stock tendered for payments of withholding taxes	(2,875)						$ (2,875)
Stock tendered for payments of withholding taxes (in shares)							(75,881)
Issuance of restricted stock awards, net of forfeitures			(8,905)				$ 8,905
Issuance of restricted stock awards, net of forfeitures (in shares)							283,820
Share-based compensation expense	12,434		12,434
Net income	79,216				79,216
Adoption of ASU 2016-02 | ASU No. 2016-02 [Member]	(26)				(26)
Foreign currency translation adjustment	(29)					(29)
Ending Balance at Sep. 30, 2019	1,044,556	$ 419	175,749	(15,458)	1,151,646	(211)	$ (267,589)
Ending Balance (in shares) at Sep. 30, 2019		41,887,088					(7,849,464)
Beginning Balance at Jun. 30, 2019	1,032,582	$ 419	170,619	(15,458)	1,125,541	(178)	$ (248,361)
Beginning Balance (in shares) at Jun. 30, 2019		41,887,088					(7,369,052)
Purchase of treasury shares	(17,716)						$ (17,716)
Purchase of treasury shares (in shares)							(444,509)
Stock tendered for payments of withholding taxes	(129)						$ (129)
Stock tendered for payments of withholding taxes (in shares)							(3,168)
Issuance of restricted stock awards, net of forfeitures			1,383				$ (1,383)
Issuance of restricted stock awards, net of forfeitures (in shares)							(32,735)
Share-based compensation expense	3,747		3,747
Net income	26,105				26,105
Foreign currency translation adjustment	(33)					(33)
Ending Balance at Sep. 30, 2019	$ 1,044,556	$ 419	$ 175,749	$ (15,458)	$ 1,151,646	$ (211)	$ (267,589)
Ending Balance (in shares) at Sep. 30, 2019		41,887,088					(7,849,464)