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Debt - Unsecured Notes Payable (Details) - USD ($)
3 Months Ended
Mar. 31, 2017
Dec. 31, 2016
Debt Instrument [Line Items]    
Total indebtedness $ 1,501,896,000 $ 1,488,718,000
Unsecured Debt [Member]    
Debt Instrument [Line Items]    
Total indebtedness 0 9,900,000
Fund IV [Member] | Line of Credit [Member] | Unsecured Debt [Member]    
Debt Instrument [Line Items]    
Maximum borrowing capacity 21,500,000.0 100,000,000.0
Repayments of debt 74,100,000  
Proceeds from (repayments of) lines of credit 10,000,000  
Long-term line of credit, noncurrent 20,400,000 94,500,000
Remaining borrowing capacity $ 1,100,000 $ 55,500,000