Stockholders' Investment (Tables)

9 Months Ended

Sep. 30, 2014

Equity [Abstract]

Schedule of changes in stockholders' investment

	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2014		2013		2014		2013
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	—		38,150		—		46,653
Issuance of common stock from share-based compensation	—		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