Annual report pursuant to Section 13 and 15(d)

Shareholders' Equity (Details)

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Shareholders' Equity (Details) (USD $)
Share data in Thousands, except Per Share data, unless otherwise specified
12 Months Ended 36 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Dec. 31, 2011
Common shares issued 66,265 65,870   66,265
Net proceeds $ 0 $ 168,880,000 $ 160,915,000  
Shares issued under Dividend Reinvestment Program, shares 200 200 100  
Weighted average issue price, dividend reinvestment program   $ 30.36 $ 28.34 $ 29.97
Net proceeds, dividend reinvestment program 5,043,000 5,286,000 2,479,000  
Common Stock, Capital Shares Reserved for Future Issuances, Value 250,000,000     250,000,000
At The Market Offering Program [Member]
       
Common shares issued   5,600 2,000  
Weighted average issue price   $ 30.34 $ 27.37  
Net proceeds   168,900,000 53,800,000  
Dividend Reinvestment Program [Member]
       
Net proceeds, dividend reinvestment program $ 5,041,000 $ 5,284,000 $ 2,478,000