Stockholders' Investment (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Stockholders' investment:
Beginning balance	$ 594,812	$ 530,764	$ 558,775	$ 500,365
Net Income	5,791	14,413	35,866	39,595
Share-based Compensation	423	614	2,044	1,750
Issuance costs from secondary stock offering	(225)	0	(225)	0
Issuance of common stock from share-based compensation	(68)	0	3,082	2,715
Excess tax benefit on share-based compensation	(16)	76	1,175	1,442
Ending balance	$ 600,717	$ 545,867	$ 600,717	$ 545,867