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Debt - Mortgage Debt (Details) - USD ($)
1 Months Ended 9 Months Ended
Jan. 31, 2018
Sep. 30, 2018
Sep. 30, 2017
Dec. 31, 2017
Debt Instrument [Line Items]        
Repayment of mortgage notes payable   $ 265,688,000 $ 344,618,000  
Mortgages        
Debt Instrument [Line Items]        
Fixed rate mortgage debt   615,537,000   $ 721,224,000
1333 Broadway | Mortgages        
Debt Instrument [Line Items]        
Fixed rate mortgage debt   $ 160,000,000   $ 66,602,000
Face amount $ 160,000,000.0      
Stated rate (as a percent) 4.21% 4.21%    
1400 Broadway | Mortgages        
Debt Instrument [Line Items]        
Repayment of mortgage notes payable $ 75,800,000