Stockholders' Investment (Tables)

6 Months Ended

Jun. 30, 2015

Equity [Abstract]

Schedule of changes in stockholders' investment

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2015		2014		2015		2014
Beginning balance	$	575,325	$	514,694	$	558,775	$	500,365
Net income	16,471		14,768		30,075		25,182
Share-based compensation	825		642		1,621		1,123
Issuance of common stock from share-based compensation	1,811		325		3,150		2,727
Excess tax benefit on share-based compensation	380		335		1,191		1,367
Ending balance	$	594,812	$	530,764	$	594,812	$	530,764