Annual report pursuant to Section 13 and 15(d)

Unsecured Lines Of Credit Payable (Repayments And Borrowings On Unsecured Lines Of Credit) (Details)

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Unsecured Lines Of Credit Payable (Repayments And Borrowings On Unsecured Lines Of Credit) (Details) (USD $)
12 Months Ended
Dec. 31, 2011
Dec. 31, 2010
Dec. 31, 2009
Line of Credit Facility [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity $ 475,000,000 $ 337,000,000 $ 337,000,000
Interest Expense 66,473,000 67,229,000 72,694,000
Line of Credit Facility, Commitment Fee Percentage 0.15% 0.225%  
Balance at December 31, 2011 (99,000,000) (100,000,000) (128,000,000)
Balance at December 31, 2011 (100,000,000) (128,000,000)  
Weighted average daily borrowings during the year 160,090,000 112,573,000 33,656,000
Maximum daily borrowings during the year 281,000,000 141,000,000 128,000,000
Weighted average interest rate during the year 1.90% 2.43% 1.62%
Weighted average interest rate at period end 0.90% 2.53% 2.79%
Credit Facility No. 1 [Member]
     
Line of Credit Facility [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity 75,000,000    
Borrowings 92,000,000    
Repayments (18,000,000)    
Interest Expense 400,000 100,000 0
Debt, Weighted Average Interest Rate 0.65% 0.71% 0.70%
Line of Credit Facility, Commitment Fee Amount 100,000 100,000 100,000
Balance at December 31, 2011 74,000,000 0  
Balance at December 31, 2011 0    
Credit Facility No. 2 [Member]
     
Line of Credit Facility [Line Items]      
Line of Credit Facility, Maximum Borrowing Capacity 400,000,000    
Borrowings 169,000,000    
Repayments (244,000,000)    
Interest Expense 2,700,000 2,700,000 500,000
Debt, Weighted Average Interest Rate 2.54% 2.65% 1.81%
Line of Credit Facility, Commitment Fee Amount 700,000 400,000 400,000
Balance at December 31, 2011 25,000,000 (100,000,000)  
Balance at December 31, 2011 $ (100,000,000)    
Federal Funds Reference Rate [Member] | Credit Facility No. 1 [Member]
     
Line of Credit Facility [Line Items]      
Variable, Interest Rate Spread, Basis Points 0.50%    
Federal Funds Reference Rate [Member] | Credit Facility No. 2 [Member]
     
Line of Credit Facility [Line Items]      
Variable, Interest Rate Spread, Basis Points 1.00%    
Current Basis Spread on Variable Rate [Member] | Credit Facility No. 1 [Member]
     
Line of Credit Facility [Line Items]      
Variable, Interest Rate Spread, Basis Points 4250.00%    
Current Basis Spread on Variable Rate [Member] | Credit Facility No. 2 [Member]
     
Line of Credit Facility [Line Items]      
Variable, Interest Rate Spread, Basis Points 12250.00%