Consolidated Statements of Shareholders' Equity (unaudited) - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid in Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]
Beginning balance, value at Dec. 31, 2017	$ 281,247	$ 601	$ 161,809	$ 258,525	$ (139,688)
Beginning balance, shares at Dec. 31, 2017		60,149,000			2,361,000
Vesting of restricted stock		$ 3	(3)
Vesting of restricted stock, shares		298,000
Stock-based compensation	27,496		27,496
Repurchases of common stock	(16,870)				$ (16,870)
Repurchases of common stock, shares					169,000
Net income	41,160			41,160
Ending balance, value at Mar. 31, 2018	333,033	$ 604	189,302	299,685	$ (156,558)
Ending balance, shares at Mar. 31, 2018		60,447,000			2,530,000
Beginning balance, value at Dec. 31, 2017	281,247	$ 601	161,809	258,525	$ (139,688)
Beginning balance, shares at Dec. 31, 2017		60,149,000			2,361,000
Net income	105,651
Ending balance, value at Sep. 30, 2018	360,377	$ 606	198,255	364,176	$ (202,660)
Ending balance, shares at Sep. 30, 2018		60,698,000			2,968,000
Beginning balance, value at Mar. 31, 2018	333,033	$ 604	189,302	299,685	$ (156,558)
Beginning balance, shares at Mar. 31, 2018		60,447,000			2,530,000
Vesting of restricted stock		$ 2	(2)
Vesting of restricted stock, shares		192,000
Stock-based compensation	3,988		3,988
Repurchases of common stock	(43,005)				$ (43,005)
Repurchases of common stock, shares					408,000
Net income	35,722			35,722
Ending balance, value at Jun. 30, 2018	329,738	$ 606	193,288	335,407	$ (199,563)
Ending balance, shares at Jun. 30, 2018		60,639,000			2,938,000
Vesting of restricted stock, shares		59,000
Stock-based compensation	4,967		4,967
Repurchases of common stock	(3,097)				$ (3,097)
Repurchases of common stock, shares					30,000
Net income	28,769			28,769
Ending balance, value at Sep. 30, 2018	360,377	$ 606	198,255	364,176	$ (202,660)
Ending balance, shares at Sep. 30, 2018		60,698,000			2,968,000
Beginning balance, value at Dec. 31, 2018	334,753	$ 607	203,680	395,590	$ (265,124)
Beginning balance, shares at Dec. 31, 2018		60,747,000			3,470,000
Vesting of restricted stock		$ 2	(2)
Vesting of restricted stock, shares		280,000
Stock-based compensation	36,187		36,187
Repurchases of common stock	(19,669)				$ (19,669)
Repurchases of common stock, shares					110,000
Net income	47,282			47,282
Ending balance, value at Mar. 31, 2019	398,553	$ 609	239,865	442,872	$ (284,793)
Ending balance, shares at Mar. 31, 2019		61,027,000			3,580,000
Beginning balance, value at Dec. 31, 2018	334,753	$ 607	203,680	395,590	$ (265,124)
Beginning balance, shares at Dec. 31, 2018		60,747,000			3,470,000
Net income	135,196
Ending balance, value at Sep. 30, 2019	474,898	$ 612	250,853	530,786	$ (307,353)
Ending balance, shares at Sep. 30, 2019		61,346,000			3,688,000
Beginning balance, value at Mar. 31, 2019	398,553	$ 609	239,865	442,872	$ (284,793)
Beginning balance, shares at Mar. 31, 2019		61,027,000			3,580,000
Vesting of restricted stock		$ 3	(3)
Vesting of restricted stock, shares		275,000
Stock-based compensation	6,207		6,207
Repurchases of common stock	(18,760)				$ (18,760)
Repurchases of common stock, shares					92,000
Net income	48,762			48,762
Ending balance, value at Jun. 30, 2019	434,762	$ 612	246,069	491,634	$ (303,553)
Ending balance, shares at Jun. 30, 2019		61,302,000			3,672,000
Vesting of restricted stock, shares		44,000
Stock-based compensation	4,784		4,784
Repurchases of common stock	(3,800)				$ (3,800)
Repurchases of common stock, shares					16,000
Net income	39,152			39,152
Ending balance, value at Sep. 30, 2019	$ 474,898	$ 612	$ 250,853	$ 530,786	$ (307,353)
Ending balance, shares at Sep. 30, 2019		61,346,000			3,688,000