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, 2013
Dec. 31, 2012
Dec. 31, 2011
Line of Credit Facility, Amount Outstanding [Roll Forward]      
Balance at December 31, 2011 $ 0 $ 99,000,000  
Balance at December 31, 2012 0 0 99,000,000
Interest expense, net of capitalized interest 63,573,000 60,627,000 61,402,000
Total revolving credit facilities at December 31 500,000,000 500,000,000 475,000,000
Balance at December 31, 2011 0 0 99,000,000
Weighted average daily borrowings during the year 61,548,000 108,589,000 160,090,000
Maximum daily borrowings during the year 135,000,000 242,000,000 281,000,000
Weighted average interest rate during the year 1.41% 1.15% 1.90%
Weighted average interest rate at period end     0.90%
Credit Facility No. 1 [Member]
     
Line of Credit Facility, Amount Outstanding [Roll Forward]      
Balance at December 31, 2011 0    
Borrowings 100,000,000    
Repayments (100,000,000)    
Balance at December 31, 2012 0 0  
Interest expense, net of capitalized interest 281,000 470,000 355,000
Facility fees 253,000 175,000 114,000
Total revolving credit facilities at December 31 100,000,000    
Balance at December 31, 2011 0 0  
Credit Facility No. 2 [Member]
     
Line of Credit Facility, Amount Outstanding [Roll Forward]      
Balance at December 31, 2011 0    
Borrowings 60,000,000    
Repayments (60,000,000)    
Balance at December 31, 2012 0 0  
Interest expense, net of capitalized interest 586,000 783,000 2,735,000
Facility fees 1,014,000 887,000 658,000
Total revolving credit facilities at December 31 400,000,000    
Balance at December 31, 2011 $ 0 $ 0