Stockholders' Investment (Details) (USD $)	3 Months Ended		0 Months Ended
	Mar. 31, 2013	Mar. 31, 2012	Jan. 04, 2013 Issuance of Equity [Member]
Stockholders' investment:
Beginning balance	$ 392,079,000	$ 295,953,000
Net Income	10,582,000	7,931,000
Other changes	9,566,000	1,160,000
Ending balance	412,227,000	305,044,000
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