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, 2014	$ 424,531	$ 106,469	$ 318,164	$ (102)
Balance, Shares at Dec. 31, 2014		26,051,000
Net income	40,470		40,470
Share-based compensation	3,399	$ 3,399
Common stock issued under stock plans, net of shares withheld for employee taxes	2,149	$ 2,149
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		208,000
Common stock repurchased	(63,953)	$ (9,119)	(54,834)
Common stock repurchased, Shares		(2,408,000)
Tax shortfall from equity awards	(292)	$ (292)
Taxes paid related to net share settlement of stock awards	(1,560)	(1,560)
Dividends accrued of $1.00, $1.02, $1.04 per share in 2015, 2016 and 2017	(25,092)		(25,092)
Other comprehensive (loss) gain	35			35
Balance at Dec. 31, 2015	379,687	$ 101,046	278,708	(67)
Balance, Shares at Dec. 31, 2015		23,851,000
Net income	6,566
Balance at Mar. 31, 2016	380,512
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
Common stock repurchased, Shares	0
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	23,948,000
Balance at Mar. 31, 2016	$ 380,512
Net income	9,079
Balance at Jun. 30, 2016	383,313
Net income	12,872
Balance at Sep. 30, 2016	390,600
Net income	9,734
Balance at Dec. 31, 2016	$ 394,287	$ 101,821	292,521	(55)
Balance, Shares at Dec. 31, 2016	23,948,000	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	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
Common stock repurchased, Shares	0
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