Unaudited Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Additional Paid in Capital [Member]	Retained Earnings [Member]	Treasury Stock [Member]
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
Beginning balance, value at Dec. 31, 2019	526,628	$ 613	257,501	576,166	$ (307,652)
Beginning balance, shares at Dec. 31, 2019		61,350,000			3,689,000
Vesting of restricted stock		$ 1	(1)
Vesting of restricted stock, shares		2,000
Stock-based compensation	18,313		18,313
Repurchases of common stock	(8,168)				$ (8,168)
Repurchases of common stock, shares					41,000
Net income	63,015			63,015
Ending balance, value at Mar. 31, 2020	599,788	$ 614	275,813	639,181	$ (315,820)
Ending balance, shares at Mar. 31, 2020		61,352,000			3,730,000
Beginning balance, value at Dec. 31, 2019	526,628	$ 613	257,501	576,166	$ (307,652)
Beginning balance, shares at Dec. 31, 2019		61,350,000			3,689,000
Net income	119,086
Ending balance, value at Sep. 30, 2020	632,399	$ 616	322,321	695,252	$ (385,790)
Ending balance, shares at Sep. 30, 2020		61,639,000			4,034,000
Beginning balance, value at Mar. 31, 2020	599,788	$ 614	275,813	639,181	$ (315,820)
Beginning balance, shares at Mar. 31, 2020		61,352,000			3,730,000
Vesting of restricted stock		$ 2	(2)
Vesting of restricted stock, shares		261,000
Stock-based compensation	24,597		24,597
Repurchases of common stock	(67,121)				$ (67,121)
Repurchases of common stock, shares					295,000
Net income	28,589			28,589
Ending balance, value at Jun. 30, 2020	585,853	$ 616	300,408	667,770	$ (382,941)
Ending balance, shares at Jun. 30, 2020		61,613,000			4,025,000
Vesting of restricted stock, shares		26,000
Stock-based compensation	21,913		21,913
Repurchases of common stock	(2,849)				$ (2,849)
Repurchases of common stock, shares					9,000
Net income	27,482			27,482
Ending balance, value at Sep. 30, 2020	$ 632,399	$ 616	$ 322,321	$ 695,252	$ (385,790)
Ending balance, shares at Sep. 30, 2020		61,639,000			4,034,000