Annual report pursuant to Section 13 and 15(d)

Unsecured Lines Of Credit Payable - Lines Of Credit Unused and Available (Details)

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Unsecured Lines Of Credit Payable - Lines Of Credit Unused and Available (Details) (USD $)
Dec. 31, 2012
Dec. 31, 2011
Dec. 31, 2010
Line of Credit Facility [Line Items]      
Committed capacity $ 500,000,000 $ 475,000,000 $ 337,000,000
Borrowings outstanding 0 99,000,000 100,000,000
Credit Facility No. 1 [Member]
     
Line of Credit Facility [Line Items]      
Committed capacity 100,000,000    
Borrowings outstanding 0 74,000,000  
Letters of credit issued (815,000)    
Unused and available 99,185,000    
Credit Facility No. 2 [Member]
     
Line of Credit Facility [Line Items]      
Committed capacity 400,000,000    
Borrowings outstanding 0 25,000,000  
Letters of credit issued 0    
Unused and available $ 400,000,000