Real Estate Assets, Net - Narrative (Details)	3 Months Ended			9 Months Ended			12 Months Ended
	Sep. 30, 2021 USD ($) property		Sep. 30, 2020 USD ($) property	Sep. 30, 2021 USD ($) property		Sep. 30, 2020 USD ($) property	Dec. 31, 2020 USD ($) property		Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)
Real Estate [Line Items]
Net investments in real estate	$ 6,097,000	[1]	$ 7,208,000	$ 6,097,000	[1]	$ 7,208,000	$ 8,526,000	[1]	$ 4,768,000	$ 8,233,000	$ 15,071,000
Property held-for-sale	6,100,000			6,100,000			7,800,000
Rental properties	$ 0			0			700,000
Real estate held for sale improvements				$ 400,000			$ 300,000
Number of properties owned | property				31			38
Number of held-for-sale properties | property				31			32
Number of rental properties | property				0			6
Number of held-for-sale residential properties disposed | property	4		7	24		40
Gain (loss) on sale of property				$ 572,000		$ 957,000
Real estate impairment	$ 100,000		$ 200,000	200,000		1,200,000
Other income
Real Estate [Line Items]
Gain (loss) on sale of property	400,000		$ 200,000	600,000		$ 1,000,000
Carrying value
Real Estate [Line Items]
Net investments in real estate	6,100,000			6,100,000			$ 8,500,000
Property held-for-sale	$ 6,097,000			$ 6,097,000			$ 7,807,000

[1] Real estate owned properties, net, are presented net of valuation allowances of $0.4 million and $1.4 million at September 30, 2021 and December 31, 2020, respectively.