Stockholders' Investment (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Stockholders' investment:
Beginning balance	$ 530,764	$ 429,973	$ 500,365	$ 392,079
Net Income	14,413	13,230	39,595	37,782
Share-based Compensation	614	451	1,750	1,155
Issuance of common stock from stock offering	0	38,150	0	46,653
Issuance of common stock from share-based compensation	0	2,176	2,715	6,311
Excess tax benefit on share-based compensation	76	3,765	1,442	3,765
Ending balance	$ 545,867	$ 487,745	$ 545,867	$ 487,745