Common Stock Summary of Common Stock (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Class of Stock [Line Items]
Common Stock, Shares, Outstanding		10,939,486	8,755,398	8,676,849	8,573,744
Common Stock Gross, Value, Issued		$ 138,949	$ 73,958	$ 71,636	$ 68,896
Common Stock Issuance Expense, Value, Issued		(4,076)	(1,613)	(1,612)	(1,610)
Shares Issued Through Performance Stock Program		34,301	20,210	31,282
Shares Issued Through Performance Stock Program, Value		1,213	824	1,077
Shares Issued Through Performance Stock Program, Value, Expense	0	0		0
Shares Issued Through Performance Stock Program, Value, Net	824	1,213		1,077
Stock Issued During Period, Shares, Dividend Reinvestment Plan		50,048	52,668	57,749
Stock Issued During Period, Value, Dividend Reinvestment Plan		1,486	1,346	1,358
Stock Issued During Period, Dividend Reinvestment Plan, Expense	0	0		0
Stock Issued During Period, Dividend Reinvestment Plan, Net	1,346	1,486		1,358
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period		23,235	5,671	14,074
Stock Issued During Period, Value, Stock Options Exercised		665	152	305
Stock Issued During Period, Stock Options Exercised, Expense	(1)	(3)		(2)
Stock Issued During Period, Stock Options Exercised, Net	151	662		303
Stock Issued During Period, Shares, Acquisitions		380,254
Stock Issued During Period, Value, Acquisitions		12,012	0	0
Stock Issued During Period, Acquisitions, Expense		(154)
Stock Issued During Period, Acquisitions, Net		11,858
Stock Issued During Period, Shares, New Issues		1,696,250
Stock Issued During Period, Value, New Issues		49,615
Stock Issued During Period, New Issues, Expense		(2,306)
Stock Issued During Period, New Issues, Net		47,309
Common Stock Without Par Value: Authorized - 25,000,000 Shares - Issued and Outstanding: 2012 - 8,848,848; 2011 - 8,755,398		$ 134,873	$ 72,345	$ 70,024	$ 67,286