Notes and Other Receivables - Allowance for credit loss (Details) - USD ($)	9 Months Ended
	Sep. 30, 2020	Jan. 01, 2020
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 0
Impact of adoption ASC 326	172,000
Provision for credit losses	(100,000)
Ending balance	172,000
Mezzanine Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0
Impact of adoption ASC 326	172,000
Provision for credit losses	25,000
Ending balance	172,000
Bridge Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0
Impact of adoption ASC 326	0
Provision for credit losses	(43,000)
Ending balance	0
Mezzanine and Bridge Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0
Impact of adoption ASC 326	172,000
Provision for credit losses	(18,000)
Ending balance	$ 172,000
Cumulative Effect, Period Of Adoption, Adjustment | Mezzanine Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Impact of adoption ASC 326		$ 147,000
Cumulative Effect, Period Of Adoption, Adjustment | Bridge Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Impact of adoption ASC 326		43,000
Cumulative Effect, Period Of Adoption, Adjustment | Mezzanine and Bridge Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Impact of adoption ASC 326		$ 190,000