Real Estate - Schedule of Property Dispositions (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain (Loss) on Sale	$ 24	$ 12,056	$ 509	$ 14,070
Disposed of by sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale Price			15,650
Gain (Loss) on Sale			509
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	$ 24	$ 12,056	$ 509	$ 14,070	$ 30,324
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
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 | 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 | 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
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
Core Portfolio | Disposal Group, Not Discontinued Operations | Pacesetter Park
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