Financial Instruments and Fair Value Measurements - Schedule of Items Measured at Fair Value on Nonrecurring Basis (Details) - USD ($)	3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2020	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2020	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019
Derivatives, Fair Value [Line Items]
Impairment charges		$ 321,000			$ 51,549,000	$ 1,721,000
Fair Value, Measurements, Nonrecurring
Derivatives, Fair Value [Line Items]
Impairment charges	$ 51,549,000			$ 0			$ 1,721,000
Fair Value, Measurements, Nonrecurring | Acadia Realty Trust
Derivatives, Fair Value [Line Items]
Impairment charges	$ 12,400,000						$ 395,000
Cortlandt Crossing, Mohegan Lake, NY | Fair Value, Measurements, Nonrecurring | Fund III | Level 3 Inputs Projections
Derivatives, Fair Value [Line Items]
Triggering Event	Reduced holding period, reduced projected operating income
Impairment charges	$ 27,402,000
Cortlandt Crossing, Mohegan Lake, NY | Fair Value, Measurements, Nonrecurring | Fund III | Acadia Realty Trust | Level 3 Inputs Projections
Derivatives, Fair Value [Line Items]
Impairment charges	$ 6,726,000
654 Broadway, New York, NY | Fair Value, Measurements, Nonrecurring | Fund III | Level 3 Inputs Projections
Derivatives, Fair Value [Line Items]
Triggering Event	Reduced holding period
Impairment charges	$ 6,398,000
654 Broadway, New York, NY | Fair Value, Measurements, Nonrecurring | Fund III | Acadia Realty Trust | Level 3 Inputs Projections
Derivatives, Fair Value [Line Items]
Impairment charges	$ 1,570,000
146 Geary Street, San Francisco, CA | Fair Value, Measurements, Nonrecurring | Fund IV | Level 3 Inputs Projections
Derivatives, Fair Value [Line Items]
Triggering Event	Reduced holding period, reduced projected operating income
Impairment charges	$ 6,718,000
146 Geary Street, San Francisco, CA | Fair Value, Measurements, Nonrecurring | Fund IV | Acadia Realty Trust | Level 3 Inputs Projections
Derivatives, Fair Value [Line Items]
Impairment charges	$ 1,553,000
801 Madison Avenue, New York, NY | Fair Value, Measurements, Nonrecurring | Fund IV | Level 3 Inputs Projections
Derivatives, Fair Value [Line Items]
Triggering Event	Reduced holding period, reduced projected operating income
Impairment charges	$ 11,031,000
801 Madison Avenue, New York, NY | Fair Value, Measurements, Nonrecurring | Fund IV | Acadia Realty Trust | Level 3 Inputs Projections
Derivatives, Fair Value [Line Items]
Impairment charges	$ 2,551,000
210 Bowery Residential Units | Fair Value, Measurements, Nonrecurring | Fund IV | Level 3 Inputs Contract Sales Price
Derivatives, Fair Value [Line Items]
Triggering Event		Reduced selling price
Impairment charges		$ 321,000
210 Bowery Residential Units | Fair Value, Measurements, Nonrecurring | Fund IV | Level 3 Inputs Offering Price
Derivatives, Fair Value [Line Items]
Triggering Event			Reduced selling price
Impairment charges			$ 1,400,000
210 Bowery Residential Units | Fair Value, Measurements, Nonrecurring | Fund IV | Acadia Realty Trust | Level 3 Inputs Contract Sales Price
Derivatives, Fair Value [Line Items]
Impairment charges		$ 74,000
210 Bowery Residential Units | Fair Value, Measurements, Nonrecurring | Fund IV | Acadia Realty Trust | Level 3 Inputs Offering Price
Derivatives, Fair Value [Line Items]
Impairment charges			$ 321,000