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Notes Payable and Unsecured Credit Facilities Schedule of maturities of long-term debt (Details) - USD ($)
$ in Thousands
Jun. 30, 2018
Dec. 31, 2017
Debt Instrument [Line Items]    
Long-term Debt, Maturities, Repayments of Principal, Remainder of Fiscal Year $ 111,290  
Long-term Debt, Maturities, Repayments of Principal in Year Two 33,044  
Long-term Debt, Maturities, Repayments of Principal in Year Three 389,867  
Long-term Debt, Maturities, Repayments of Principal in Year Four 328,351  
Long-term Debt, Maturities, Repayments of Principal in Year Five 732,647  
Long-term Debt, Maturities, Repayments of Principal after Year Five 2,256,272  
Debt Instrument, Unamortized Discount (Premium), Net 19,971  
Long-term Debt 3,831,500 $ 3,594,977
Unsecured Debt [Member]    
Debt Instrument [Line Items]    
Long-term Debt 2,474,126 $ 2,325,656
Scheduled Principal Payments [Member] | Mortgages [Member]    
Debt Instrument [Line Items]    
Long-term Debt, Maturities, Repayments of Principal, Remainder of Fiscal Year 5,173  
Long-term Debt, Maturities, Repayments of Principal in Year Two 9,519  
Long-term Debt, Maturities, Repayments of Principal in Year Three 11,287  
Long-term Debt, Maturities, Repayments of Principal in Year Four 11,600  
Long-term Debt, Maturities, Repayments of Principal in Year Five 11,799  
Long-term Debt, Maturities, Repayments of Principal after Year Five 37,056  
Debt Instrument, Unamortized Discount (Premium), Net 0  
Long-term Debt 86,434  
Mortgage Loan Maturities [Member] | Mortgages [Member]    
Debt Instrument [Line Items]    
Long-term Debt, Maturities, Repayments of Principal, Remainder of Fiscal Year 106,117  
Long-term Debt, Maturities, Repayments of Principal in Year Two 23,525  
Long-term Debt, Maturities, Repayments of Principal in Year Three 78,580  
Long-term Debt, Maturities, Repayments of Principal in Year Four 66,751  
Long-term Debt, Maturities, Repayments of Principal in Year Five 5,848  
Long-term Debt, Maturities, Repayments of Principal after Year Five 269,216  
Debt Instrument, Unamortized Discount (Premium), Net (7,405)  
Long-term Debt 557,442  
Unsecured Maturities [Member] | Unsecured Debt [Member]    
Debt Instrument [Line Items]    
Long-term Debt, Maturities, Repayments of Principal, Remainder of Fiscal Year 0  
Long-term Debt, Maturities, Repayments of Principal in Year Two 0  
Long-term Debt, Maturities, Repayments of Principal in Year Three 300,000  
Long-term Debt, Maturities, Repayments of Principal in Year Four 250,000  
Long-term Debt, Maturities, Repayments of Principal in Year Five 715,000  
Long-term Debt, Maturities, Repayments of Principal after Year Five 1,950,000  
Debt Instrument, Unamortized Discount (Premium), Net 27,376  
Long-term Debt $ 3,187,624