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, 2012
Dec. 31, 2011
Dec. 31, 2010
Line of Credit Facility, Amount Outstanding [Roll Forward]      
Balance at December 31, 2011 $ 99,000,000 $ 100,000,000  
Balance at December 31, 2012 0 99,000,000 100,000,000
Interest expense, net of capitalized interest 64,697,000 66,214,000 66,965,000
Total revolving credit facilities at December 31 500,000,000 475,000,000 337,000,000
Balance at December 31, 2011 0 99,000,000 100,000,000
Weighted average daily borrowings during the year 108,589,000 160,090,000 112,573,000
Maximum daily borrowings during the year 242,000,000 281,000,000 141,000,000
Weighted average interest rate during the year 1.15% 1.90% 2.43%
Weighted average interest rate at period end   0.90% 2.53%
Credit Facility No. 1 [Member]
     
Line of Credit Facility, Amount Outstanding [Roll Forward]      
Balance at December 31, 2011 74,000,000    
Borrowings 0    
Repayments (74,000,000)    
Balance at December 31, 2012 0 74,000,000  
Interest expense, net of capitalized interest 470,000 355,000 91,000
Facility fees 175,000 114,000 114,000
Total revolving credit facilities at December 31 100,000,000    
Balance at December 31, 2011 0 74,000,000  
Credit Facility No. 2 [Member]
     
Line of Credit Facility, Amount Outstanding [Roll Forward]      
Balance at December 31, 2011 25,000,000    
Borrowings 158,000,000    
Repayments (183,000,000)    
Balance at December 31, 2012 0 25,000,000  
Interest expense, net of capitalized interest 783,000 2,735,000 2,684,000
Facility fees 887,000 658,000 398,000
Total revolving credit facilities at December 31 400,000,000    
Balance at December 31, 2011 $ 0 $ 25,000,000