Notes and Other Receivables - Allowance for credit loss (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	$ 0
Impact of adoption ASC 326	0	$ 0
Provision for credit losses	(687)	0	$ 0
Ending balance	751	0
Mezzanine Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0
Impact of adoption ASC 326	751	0
Provision for credit losses	604
Ending balance	751	0
Bridge Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0
Impact of adoption ASC 326	0	0
Provision for credit losses	(43)
Ending balance	0	0
Mezzanine and Bridge Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	0
Impact of adoption ASC 326	751	0
Provision for credit losses	561
Ending balance	751	0
Cumulative Effect, Period of Adoption, Adjustment | Mezzanine Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	147
Impact of adoption ASC 326	147	147
Ending balance		147
Cumulative Effect, Period of Adoption, Adjustment | Bridge Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	43
Impact of adoption ASC 326	43	43
Ending balance		43
Cumulative Effect, Period of Adoption, Adjustment | Mezzanine and Bridge Loans
Financing Receivable, Allowance for Credit Loss [Roll Forward]
Beginning balance	190
Impact of adoption ASC 326	$ 190	190
Ending balance		$ 190