Real Estate - Schedule of Property Dispositions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2021		Dec. 31, 2020	Dec. 31, 2019
Income Statement, Balance Sheet and Additional Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Gain Loss On Sale Of Properties	$ 10,521		$ 683	$ 30,324
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	109,326
Gain Loss On Sale Of Properties	$ 9,787	[1]	$ 683	$ 30,324
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 Loss On Sale Of Properties	$ 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 Loss On Sale Of Properties			$ 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 Of Properties			$ 86
Core Portfolio | Disposed of by sale | 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 Of Properties				$ 16,771
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 Loss On Sale Of Properties	$ 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 Loss On Sale Of Properties			$ 0
Fund III | Disposed of by sale | 3104 M Street - Washington, DC
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 Of Properties				$ 2,014
Fund III | Disposed of by sale | 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 Of Properties				$ 2,562
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 Loss On Sale Of Properties	$ 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 Loss On Sale Of Properties			$ 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 Of Properties			$ 24
Fund IV | Disposed of by sale | 10 Bowery - 3 Residential Condos - New York, NY
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 Of Properties				$ (242)
Fund IV | Disposed of by sale | 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 Of Properties				$ 2,075
Fund IV | Disposed of by sale | 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 Of Properties				$ 7,144

[1] Does not include the gain on lease termination of $0.7 million related to the Fund IV lease at 110 University Place (Note 12).