Stockholders' Investment (Details) (USD $)	3 Months Ended		6 Months Ended		0 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012	Jan. 04, 2013 Issuance of Equity [Member]
Stockholders' investment:
Beginning balance	$ 412,227,000	$ 305,044,000	$ 392,079,000	$ 295,953,000
Net Income	13,970,000	10,200,000	24,552,000	18,131,000
Other changes	3,776,000	280,000	13,342,000	1,440,000
Ending balance	429,973,000	315,524,000	429,973,000	315,524,000
Options to purchase additonal shares					525,000
Exercise price of shares					17.25
Proceeds from Sale of Shares, Net of Underwriting Discount and Estimated Expenses					$ 8,500,000