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, 2011	$ 333,142	$ 74,878	$ 258,264
Balance, Shares at Dec. 31, 2011		24,576
Net income	44,777		44,777
Share-based compensation	3,840	3,840
Common stock issued under stock plans, net of shares withheld for employee taxes	5,841	5,841
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		355
Excess tax benefit from equity awards	1,033	1,033
Taxes paid related to net share settlement of stock awards	(250)	(250)
Dividends accrued of $0.94, $0.96, $0.98 per share in 2012,2013 and 2014	(23,645)		(23,645)
Balance at Dec. 31, 2012	364,738	85,342	279,396
Balance, Shares at Dec. 31, 2012		24,931
Net income	43,397		43,397
Share-based compensation	3,680	3,680
Common stock issued under stock plans, net of shares withheld for employee taxes	15,067	15,067
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		826
Excess tax benefit from equity awards	1,329	1,329
Taxes paid related to net share settlement of stock awards	(2,395)	(2,395)
Dividends accrued of $0.94, $0.96, $0.98 per share in 2012,2013 and 2014	(24,755)		(24,755)
Other comprehensive loss	(31)			(31)
Balance at Dec. 31, 2013	401,030	103,023	298,038	(31)
Balance, Shares at Dec. 31, 2013	25,757	25,757
Balance at Sep. 30, 2013	395,694
Net income	11,761
Other comprehensive loss	(31)			(31)
Balance at Dec. 31, 2013	401,030	103,023	298,038	(31)
Balance, Shares at Dec. 31, 2013	25,757	25,757
Net income	45,709		45,709
Share-based compensation	3,854	3,854
Common stock issued under stock plans, net of shares withheld for employee taxes	1,729	1,729
Common stock issued under stock Plans, net of shares withheld for employee taxes, Shares		294
Excess tax benefit from equity awards	1,822	1,822
Taxes paid related to net share settlement of stock awards	(3,959)	(3,959)
Dividends accrued of $0.94, $0.96, $0.98 per share in 2012,2013 and 2014	(25,583)		(25,583)
Other comprehensive loss	(102)			(71)
Balance at Dec. 31, 2014	424,531	106,469	318,164	(102)
Balance, Shares at Dec. 31, 2014	26,051	26,051
Balance at Sep. 30, 2014	414,519
Net income	13,887
Other comprehensive loss	(102)			(71)
Balance at Dec. 31, 2014	$ 424,531			$ (102)
Balance, Shares at Dec. 31, 2014	26,051