Loans Payable and Secured Line of Credit - Additional Information (Details)				1 Months Ended					6 Months Ended	12 Months Ended
	Jun. 10, 2021 USD ($)	Apr. 05, 2020 USD ($)	Jan. 15, 2020 USD ($) $ / shares	Jun. 30, 2021 USD ($) item $ / shares	Dec. 31, 2020 USD ($) item $ / shares	Jun. 30, 2020 USD ($)	Jun. 30, 2018	May 31, 2018 USD ($)	Jun. 30, 2021 USD ($) item $ / shares	Dec. 31, 2020 USD ($) $ / shares	Dec. 31, 2019 USD ($) $ / shares	Jul. 01, 2021 USD ($)	Dec. 22, 2020 $ / shares	Apr. 06, 2020 USD ($)	Jan. 31, 2020	Dec. 31, 2017 USD ($) ft² item
Loans Payable				$ 215,193,000	$ 197,330,000				$ 215,193,000	$ 197,330,000
Warrants, Exercise Price (in dollars per share) | $ / shares				$ 4.50					$ 4.50		$ 6.50		$ 4.50
Liquidity Requirement		$ 15,000,000.0												$ 10,000,000.0
Secured Debt				$ 8,950,000	7,747,000				$ 8,950,000	7,747,000
Equity Method Investments				$ 5,100,000					$ 5,100,000
250 North 10th
Equity Method Investment, Ownership Percentage				10.00%					10.00%						10.00%
Sterling National Bank [Member]
Line of Credit Facility, Maximum Borrowing Capacity				$ 12,750,000					$ 12,750,000
Debt Instrument, Interest Rate Terms									200 basis points over the 30-day LIBOR, now bears interest at the prime rate, currently 3.25%.  The secured line of credit
Landmarked Robert And Anne Dickey House [Member]
Area of Real Estate Property | ft²																7,500
Mezzanine Loan | RCG LV Debt VI REIT LLC [Member]
Debt Instrument, Face Amount								$ 15,400,000
Letter of Credit [Member]
Loans Payable				$ 8,950,000	$ 7,750,000				$ 8,950,000	$ 7,750,000
Debt Instrument, Interest Rate, Effective Percentage				3.25%	2.14%				3.25%	2.14%
Loans Payable 2018, 237 11th Loans
Debt Instrument, Interest Rate Terms								bearing interest at a blended average rate of 3.72% over the 30-day LIBOR, each with a one-year extension option upon satisfaction of certain conditions
Interest rate basis (as a percent)						0.50%
Delayed draw facility						$ 4,250,000
Debt Instrument, Interest Rate, Increase (Decrease)							0.25%
Exit fee (as a percent)						2.25%
Debt Instrument, Face Amount								$ 67,800,000
Debt Instrument Exit Fee Rate, Increase (Decrease)						0.50%
Loans Payable 2018, 237 11th Loans | Loans Payable [Member]
Repayment of loans and secured line of credit	$ 56,400,000
Debt Instrument, Exit Fees	$ 567,000
Loans Payable 2018, 237 11th Loans | Minimum [Member]
Exit fee (as a percent)						1.00%
237 11th Loan
Debt Instrument, Face Amount								$ 52,400,000
Mezzanine Loan
Term of the debt					3 years
Debt Instrument Blended Effective Interest Rate										9.44%
Collateral for loan, equity interests in subsidiaries (as a percent)					100.00%					100.00%
Debt Instrument, Face Amount					$ 7,500,000					$ 7,500,000
Debt Instrument, Number of Extensions | item					2
Debt Instrument , Extension Term					1 year
Partner Loan
Term of the debt			4 years
Loans Payable				$ 5,900,000	$ 5,900,000				$ 5,900,000	5,900,000
Debt Instrument, Face Amount			$ 5,900,000
Debt Instrument, Interest Rate, Effective Percentage			7.00%
Debt Instrument, Interest Rate, Stated Percentage			7.00%
Threshold Maximum Common Stock Price Per Share, To Trigger Prepayment Of Debt Instrument | $ / shares			$ 6.50
250 North 10th Loan
Term of the debt			15 years
Debt Instrument, Face Amount			$ 82,750,000
Debt Instrument, Interest Rate, Stated Percentage			3.39%
New 237 11th Loans
Term of the debt				2 years
Debt Instrument, Number of Extensions | item				3
Debt Instrument , Extension Term				1 year
Debt Instrument, Interest Rate, Stated Percentage				3.05%					3.05%
New Senior Loan
Debt Instrument, Face Amount				$ 50,000,000.0					$ 50,000,000.0
Initial advance at closing				48,500,000					48,500,000
Proceeds From Debt Reserved To Cover Debt Service, Operating Expense Shortfall And Leasing Related Costs				1,500,000
New Mezzanine Loan
Debt Instrument, Face Amount				10,000,000					10,000,000
Initial advance at closing				10,000,000.0					10,000,000.0
Interest Rate Cap Agreement New 237 11 Loan
Debt Instrument, Unamortized Premium				$ 32,500					$ 32,500
Debt Instrument, Interest Rate, Stated Percentage				2.50%					2.50%
Derivative, Notional Amount				$ 60,000,000.0					$ 60,000,000.0
Corporate Credit Facility
Line of Credit Facility, Maximum Borrowing Capacity											$ 70,000,000.0
Increase in maximum borrowing capacity											$ 25,000,000.0
Accrued interest				2,500,000	1,500,000				2,500,000	1,500,000
Deferred finance fees				3,400,000	3,900,000				3,400,000	3,900,000
Term of the debt											32 months
Loans Payable				35,750,000	35,750,000				$ 35,750,000	$ 35,750,000
Debt Instrument, Interest Rate Terms											a rate per annum equal to the sum of (i) 5.25% and (ii) a scheduled interest rate of 4% (the “Cash Pay Interest Rate”) which increases by 0.125% every six-month period from the Closing Date, subject to increase during the extension periods.
Debt Instrument, Interest Rate, Increase (Decrease)										0.125%
Debt Instrument, Interest Rate, Basis for Effective Rate									9.63%	9.63%
Commitment fee				$ 1,850,000	$ 2,450,000				$ 1,850,000	$ 2,450,000
Commitment fee, payable on the initial draw (as a percent)				50.00%					50.00%
Commitment fee, payable on subsequent draws (as a percent)				50.00%					50.00%
Exit fee (as a percent)				1.00%					1.00%
Multiple On Invested Capital (as a percent)				130.00%					130.00%
Collateral for loan, equity interests in subsidiaries (as a percent)				100.00%					100.00%
Common stock repurchase through proceeds from debt, amount authorized				$ 2,000,000.0					$ 2,000,000.0
Common stock repurchase through other sources of cash, amount authorized				$ 1,500,000					$ 1,500,000
Threshold minimum loan outstanding (as a percent)				50.00%					50.00%
Number of board members who can be appointed by the lender | item				1					1
Maximum number of committees of the board, that the Designee can sit | item				3					3
Reduction in commitments made by CCF									$ 7,500,000
Warrants, Exercise Price (in dollars per share) | $ / shares				$ 4.50	$ 6.50				$ 4.50	$ 6.50
Corporate Credit Facility | Greenwich Ny 77 [Member]
Net cash proceeds of residential condominium sales (as a percent)				70.00%					70.00%
Corporate Credit Facility | Minimum [Member]
Net cash proceeds of residential condominium sales (as a percent)				90.00%					90.00%
Corporate Credit Facility | Maximum [Member]
Net cash proceeds of residential condominium sales (as a percent)				100.00%					100.00%
Corporate Credit Facility | PIK Interest Rate
Interest rate basis (as a percent)											5.25%
Corporate Credit Facility | Cash Pay Interest Rate
Interest rate basis (as a percent)											4.00%
Greenwich Construction Loan
Term of the debt									4 years
Loans Payable				$ 151,800,000	$ 139,000,000.0				$ 151,800,000	$ 139,000,000.0
Debt Instrument, Face Amount																$ 189,500,000
Debt Instrument, Interest Rate, Effective Percentage				9.25%	9.25%				9.25%	9.25%
Number of Real Estate Properties | item																90
Debt Instrument, Description									LIBOR plus 8.25% and (ii) 9.25%
Liquidity Requirement		15,000,000.0												$ 10,000,000.0
Repayment of loans and secured line of credit		$ 8,000,000.0							$ 8,000,000.0
Maximum Unrestricted Cash on Liquidity Percent		50.00%
Debt Instrument, Unamortized Premium																$ 393,000
Debt Instrument, Interest Rate, Stated Percentage																2.50%
Derivative, Notional Amount																$ 189,500,000
Debt covenant, liquidity requirement, not met				$ 8,000,000.0					$ 8,000,000.0			$ 8,000,000.0
Debt covenant, required deposit to interest reserve, not met												$ 2,500,000
Greenwich Construction Loan | Construction Facility is Repaid in Between July1, 2021 And September 30, 2021 [Member]
Percentage of minimum multiple fee									40.00%