Annual report pursuant to Section 13 and 15(d)

Discontinued Operations (Narrative) (Details)

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Discontinued Operations (Narrative) (Details)
 1 Months Ended 12 Months Ended
Jan. 31, 2022
USD ($)
Dec. 31, 2021
USD ($)
ft²
item
Dec. 31, 2020
USD ($)
ft²
item
Dec. 31, 2019
USD ($)
Dec. 19, 2019
ft²
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Area of property (in square feet) | ft²   3,049,313 2,725,639    
Unrealized gains (losses) on disposition of rental property   $ 569,000 $ (14,040,000) $ (141,266,000)  
Proceeds from the sale of property   $ 52,391,000 $ 64,947,000 $ 825,613,000  
Suburban Office Portfolio [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Area of property (in square feet) | ft²         6,600,000
Disposal Group, Not Discontinued Operations [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Number of properties sold | item   16 20    
Sales proceeds   $ 711,270,000 $ 370,754,000    
Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member] | Suburban Office Properties [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Area of property (in square feet) | ft²   6,300,000      
Gain (loss) on sale of property   $ 1,000,000.0      
Subsequent Event [Member] | Disposal Group, Disposed of by Sale, Not Discontinued Operations [Member]          
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]          
Proceeds from the sale of property $ 210,000,000