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Disposed Properties - Narrative (Details) - USD ($)
12 Months Ended
Aug. 30, 2017
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Proceeds from sale and transfer of investment properties, net   $ 430,514,000 $ 233,686,000 $ 1,580,994,000
Gross Disposition Price   $ 466,175,000 244,066,000 1,924,350,000
Gain on sale of properties, net     $ 10,115,000 $ 236,256,000
Discontinued Operations, Disposed of by Sale | Worldgate Plaza        
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]        
Gross Disposition Price $ 53,500,000