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, 2015	$ 379,687	$ 101,046	$ 278,708	$ (67)
Balance, Shares at Dec. 31, 2015		23,851,000
Net income	38,251		38,251
Share-based compensation	3,091	$ 3,091
Common stock issued under stock plans, net of shares withheld for employee taxes	37	$ 37
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		97,000
Tax shortfall from equity awards	(1,066)	$ (1,066)
Taxes paid related to net share settlement of stock awards	(1,287)	(1,287)
Dividends accrued of $1.00, $1.02, $1.04 per share in 2015, 2016 and 2017	(24,438)		(24,438)
Other comprehensive (loss) gain	12			12
Balance at Dec. 31, 2016	394,287	$ 101,821	292,521	(55)
Balance, Shares at Dec. 31, 2016		23,948,000
Net income	7,973
Balance at Mar. 31, 2017	396,625
Balance at Dec. 31, 2016	394,287	$ 101,821	292,521	(55)
Balance, Shares at Dec. 31, 2016		23,948,000
Net income	153,920		153,920
Share-based compensation	3,198	$ 3,198
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		104,000
Taxes paid related to net share settlement of stock awards	(2,072)	$ (2,072)
Dividends accrued of $1.00, $1.02, $1.04 per share in 2015, 2016 and 2017	(25,036)		(25,036)
Other comprehensive (loss) gain	(113)			(113)
Balance at Dec. 31, 2017	$ 524,184	$ 102,947	421,405	(168)
Balance, Shares at Dec. 31, 2017	24,052,000	24,052,000
Balance at Mar. 31, 2017	$ 396,625
Net income	11,461
Balance at Jun. 30, 2017	402,365
Net income	16,762
Balance at Sep. 30, 2017	412,782
Net income	117,724
Balance at Dec. 31, 2017	$ 524,184	$ 102,947	421,405	(168)
Balance, Shares at Dec. 31, 2017	24,052,000	24,052,000
Net income	$ 14,466
Balance at Mar. 31, 2018	530,284
Balance at Dec. 31, 2017	$ 524,184	$ 102,947	421,405	(168)
Balance, Shares at Dec. 31, 2017	24,052,000	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 of $1.00, $1.02, $1.04 per share in 2015, 2016 and 2017	(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	24,182,000
Balance at Mar. 31, 2018	$ 530,284
Net income	15,912
Balance at Jun. 30, 2018	537,195
Net income	24,779
Balance at Sep. 30, 2018	554,547
Net income	24,249
Balance at Dec. 31, 2018	$ 571,535	$ 103,801	$ 467,783	$ (49)
Balance, Shares at Dec. 31, 2018	24,182,000	24,182,000