Consolidated Statements of Shareholders' Equity - USD ($) $ in Thousands	Total	Common Stock [Member]	Retained Earnings [Member]	Accumulated Other Comprehensive Income (Loss) [Member]
Balance at Dec. 31, 2017	$ 524,184	$ 102,947	$ 421,405	$ (168)
Balance, Shares at Dec. 31, 2017		24,052,000
Net income	79,406		79,406
Share-based compensation	4,111	$ 4,111
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		130,000
Taxes paid related to net share settlement of stock awards	(3,257)	$ (3,257)
Dividends accrued at $1.36, $1.50 and $1.68 per share in 2018, 2019 and 2020	(33,028)		(33,028)
Other comprehensive (loss) gain	119			119
Balance at Dec. 31, 2018	571,535	$ 103,801	467,783	(49)
Balance, Shares at Dec. 31, 2018		24,182,000
Net income	18,449
Balance at Mar. 31, 2019	580,643
Balance at Dec. 31, 2018	571,535	$ 103,801	467,783	(49)
Balance, Shares at Dec. 31, 2018		24,182,000
Net income	96,806		96,806
Share-based compensation	5,892	$ 5,892
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		114,000
Taxes paid related to net share settlement of stock awards	(3,333)	$ (3,333)
Dividends accrued at $1.36, $1.50 and $1.68 per share in 2018, 2019 and 2020	(36,843)		(36,843)
Other comprehensive (loss) gain	(21)			(21)
Balance at Dec. 31, 2019	$ 634,036	$ 106,360	527,746	(70)
Balance, Shares at Dec. 31, 2019	24,296,000	24,296,000
Balance at Mar. 31, 2019	$ 580,643
Net income	19,488
Balance at Jun. 30, 2019	592,309
Net income	32,468
Balance at Sep. 30, 2019	616,715
Net income	26,401
Balance at Dec. 31, 2019	$ 634,036	$ 106,360	527,746	(70)
Balance, Shares at Dec. 31, 2019	24,296,000	24,296,000
Net income	$ 20,159
Balance at Mar. 31, 2020	637,604
Balance at Dec. 31, 2019	$ 634,036	$ 106,360	527,746	(70)
Balance, Shares at Dec. 31, 2019	24,296,000	24,296,000
Net income	$ 101,984		101,984
Share-based compensation	5,549	$ 5,549
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		114,000
Repurchased common stock	(13,617)	$ (1,244)	(12,373)
Repurchased common stock, Shares		(282,000)
Taxes paid related to net share settlement of stock awards	(4,376)	$ (4,376)
Dividends accrued at $1.36, $1.50 and $1.68 per share in 2018, 2019 and 2020	(40,938)		(40,938)
Other comprehensive (loss) gain	(34)			(34)
Balance at Dec. 31, 2020	$ 682,604	$ 106,289	576,419	(104)
Balance, Shares at Dec. 31, 2020	24,128,000	24,128,000
Balance at Mar. 31, 2020	$ 637,604
Net income	22,549
Balance at Jun. 30, 2020	643,836
Net income	28,101
Balance at Sep. 30, 2020	661,614
Net income	31,175
Balance at Dec. 31, 2020	$ 682,604	$ 106,289	$ 576,419	$ (104)
Balance, Shares at Dec. 31, 2020	24,128,000	24,128,000