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Debt - Mortgage Payable - Additional Information (Details)			3 Months Ended	6 Months Ended	12 Months Ended
	Jun. 24, 2020 USD ($)	Nov. 09, 2017 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) property loan	Jun. 30, 2020 USD ($) property	Dec. 31, 2019 USD ($) property loan	Dec. 31, 2012 USD ($)
Debt Instrument [Line Items]
Fund mortgages aggregate outstanding balances				$ 134,692,000	$ 134,692,000	$ 114,943,000
Fund loans aggregating				1,700,000	$ 1,700,000
Scheduled principal payment				$ 2,700,000
Mortgages
Debt Instrument [Line Items]
Scheduled principal payment						5,900,000
Borrowings, amount						$ 258,900,000
Number of properties collateralized \| property				43	43	44
Mortgages \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate						1.70%
Mortgages \| Maturing from May 2019 to January 2021
Debt Instrument [Line Items]
Number of mortgage loans \| loan						3
Borrowings, amount						$ 69,000,000.0
Mortgages \| Maturing from May 2019 to January 2021 \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate						2.08%
Mortgages \| Maturing from April 2022 though December 2024.
Debt Instrument [Line Items]
Borrowings, amount						$ 71,800,000
Mortgages \| Maturing from April 2022 though December 2024. \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate						1.86%
Mortgages \| Maturities Ranging from November 2019 through January 2020
Debt Instrument [Line Items]
Number of mortgage loans \| loan						3
Borrowings, amount						$ 135,900,000
Mortgages \| Maturities Ranging from November 2019 through January 2020 \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate						3.65%
Mortgages \| Maturing Ranging from December 2020 though May 2021
Debt Instrument [Line Items]
Borrowings, amount						$ 125,300,000
Mortgages \| Maturing Ranging from December 2020 though May 2021 \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate						2.96%
Mortgages \| Interest Rate Swaps
Debt Instrument [Line Items]
Derivative, notional amount						$ 283,600,000
Derivative weighted average interest rate						1.78%
Fund V \| Mortgages
Debt Instrument [Line Items]
Number of mortgage loans \| loan						5
Fund V \| Mortgages \| Subscription Line
Debt Instrument [Line Items]
Number of mortgage loans \| loan				5
Fund mortgages aggregate outstanding balances				$ 304,800,000	$ 304,800,000
Fund IV \| Mortgages
Debt Instrument [Line Items]
Number of mortgage loans \| loan						3
Holdback mortgage loan				23,800,000	23,800,000
Holdback mortgage loan outstanding				18,900,000	18,900,000
Holdback mortgage loan drew an additional				1,000,000.0	1,000,000.0
Repayments of debt						$ 38,200,000
Fund IV \| Mortgages \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate						2.35%
Fund IV \| Mortgages One \| Colonie Plaza
Debt Instrument [Line Items]
Repayments of debt				$ 11,600,000
Fund IV \| Mortgages Two
Debt Instrument [Line Items]
Repayments of debt						$ 18,400,000
Fund II \| New Market Tax Credit
Debt Instrument [Line Items]
Percentage of tax credits on qualifying invested equity loans			39.00%
Transfer of tax credit transactions to investors exchange amount							$ 5,200,000
Redemption of loan expiration period				7 years
Fund II \| New Market Tax Credit \| Other Income
Debt Instrument [Line Items]
Income recognized from tax credit transactions			$ 5,000,000.0			$ 1,400,000
Fund II \| City Point Project \| New Market Tax Credit
Debt Instrument [Line Items]
Income recognized from tax credit transactions			$ 5,000,000.0
Fund II \| Mortgages
Debt Instrument [Line Items]
Number of mortgage loans \| loan						1
Fund III \| Mortgages
Debt Instrument [Line Items]
Repayments of debt						$ 4,700,000
Fund III \| Mortgages \| LIBOR
Debt Instrument [Line Items]
Basis spread on variable rate						0.50%
Fund III \| Mortgages One
Debt Instrument [Line Items]
Repayments of debt						$ 9,800,000
Core Portfolio
Debt Instrument [Line Items]
Derivative, notional amount				$ 562,035,000	562,035,000
Core Portfolio \| Mortgages \| Brandywine Portfolio
Debt Instrument [Line Items]
Debt default, amount				$ 26,300,000	$ 26,300,000	$ 26,300,000
Debt instrument, maturity date				Jul. 01, 2016
Interest rate, stated percentage				6.00%	6.00%
Debt default interest				5.00%	5.00%
Claim amount exclusive of accruing interest, default interest, late charges, and fees and costs		$ 33,000,000.0
Increase the alleged balance	$ 46,800,000
Default interest	300,000
Attorneys fees	200,000
Suspense funds	1,500,000				$ 1,300,000
Parties jointly and severally liable amount	33,200,000
Default interest per diem	$ 8,017
Legal and other costs					6,800,000
Proportionate share				$ 1,500,000	1,500,000
Interest Rate Swaps
Debt Instrument [Line Items]
Derivative, notional amount				$ 50,000,000.0	$ 50,000,000.0