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 1 Months Ended 12 Months Ended
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Jun. 30, 2012
At The Market Offering Program [Member]
Dec. 31, 2010
At The Market Offering Program [Member]
Financing Agreements [Line Items]          
Issuance and sale of stock, term       36 months  
Common stock reserved for future issuance $ 250,000,000        
Common shares issued 66,437 66,265     5,645
Weighted average issue price         $ 30.34
Net proceeds 0 0 168,880,000   168,881,000
Shares issued under Dividend Reinvestment Program, shares 55 171 175    
Weighted average issue price, dividend reinvestment program $ 29.67 $ 29.97 $ 30.36    
Net proceeds, dividend reinvestment program $ 1,316,000 $ 5,043,000 $ 5,286,000