Real Estate - Schedule of Property Dispositions (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended			12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021		Sep. 30, 2020	Dec. 31, 2020
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain on Sale	$ 0	$ 24	$ 10,521		$ 509
Disposed of by sale
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Sale Price			66,325	[1]		$ 22,413
Gain on Sale			$ 9,787	[1]		$ 683
Core Portfolio | Disposed of by sale | 60 Orange St - Bloomfield, NJ.
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold			Jan. 29, 2021
Sale Price			$ 16,400
Gain on Sale			$ 4,612
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 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 on Sale						$ 86
Fund IV | Disposed of by sale | NE Grocer Portfolio (Selected Assets) - Maine
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold			Jun. 18, 2021
Sale Price			$ 39,925
Gain on Sale			$ 5,064
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 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 on Sale						$ 24
Fund III | Disposed of by sale | 654 Broadway - New York, NY
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Date Sold			May 19, 2021
Sale Price			$ 10,000
Gain on Sale			$ 111
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
Gain on Sale						$ 0

[1] Does not include the g