Real Estate - Schedule of Property Dispositions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (Loss) on Sale	$ 683	$ 30,324	$ 5,140
Disposed of by sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale Price	22,413
Gain (Loss) on Sale	683
Disposal Group, Not Discontinued Operations
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale Price		109,326
Gain (Loss) on Sale	$ 683	$ 30,324	$ 5,140
Core Portfolio | Disposed of by sale | 163 Highland Ave. (Easement) - Needham, MA
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold	Mar. 19, 2020
Sale Price	$ 238
Gain (Loss) on Sale	$ 88
Core Portfolio | Disposed of by sale | Union Township (Parcel) - New Castle, PA
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold	Dec. 11, 2020
Sale Price	$ 200
Gain (Loss) on Sale	$ 86
Core Portfolio | Disposal Group, Not Discontinued Operations | Pacesetter Park - Pomona, NY
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold		Oct. 28, 2019
Sale Price		$ 22,550
Gain (Loss) on Sale		$ 16,771
Fund III | Disposed of by sale | Cortlandt Crossing (Sewer Project and Retention Pond) - Cortlandt, NY
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold	Nov. 30, 2020
Sale Price	$ 6,325
Fund III | Disposal Group, Not Discontinued Operations | A3104 M St. Washington D C
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold		Jan. 24, 2019
Sale Price		$ 10,500
Gain (Loss) on Sale		$ 2,014
Fund III | Disposal Group, Not Discontinued Operations | 3780-3858 Nostrand Avenue - New York, NY
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold		Aug. 22, 2019
Sale Price		$ 27,650
Gain (Loss) on Sale		$ 2,562
Fund IV | Disposed of by sale | Colonie Plaza - Albany, NY
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold	Apr. 13, 2020
Sale Price	$ 15,250
Gain (Loss) on Sale	$ 485
Fund IV | Disposed of by sale | Airport Mall (Parcel) - Bangor, ME
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold	Sep. 10, 2020
Sale Price	$ 400
Gain (Loss) on Sale	$ 24
Fund IV | Disposal Group, Not Discontinued Operations | 210 Bowery - 3 Residential Condos - New York, NY | Scenario One
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold		May 17, 2019
Sale Price		$ 8,826
Gain (Loss) on Sale		$ (242)
Fund IV | Disposal Group, Not Discontinued Operations | 210 Bowery - 3 Residential Condos - New York, NY | Scenario Two
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold		Sep. 23, 2019
Fund IV | Disposal Group, Not Discontinued Operations | 210 Bowery - 3 Residential Condos - New York, NY | Scenario Three
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold		Nov. 07, 2019
Fund IV | Disposal Group, Not Discontinued Operations | JFK Plaza - Waterville, ME
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold		Jul. 24, 2019
Sale Price		$ 7,800
Gain (Loss) on Sale		$ 2,075
Fund IV | Disposal Group, Not Discontinued Operations | 938 W North Avenue - Chicago, IL
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold		Sep. 27, 2019
Sale Price		$ 32,000
Gain (Loss) on Sale		$ 7,144