Stockholders' Investment (Tables)

6 Months Ended

Jun. 30, 2013

Equity [Abstract]

Schedule of changes in stockholders' investment

	Three Months Ended June 30,				Six Months Ended June 30,
	2013		2012		2013		2012
Beginning balance	$	412,227	$	305,044	$	392,079	$	295,953
Net income	13,970		10,200		24,552		18,131
Other changes	3,776		280		13,342		1,440
Ending balance	$	429,973	$	315,524	$	429,973	$	315,524