Real Estate Loans, Notes Receivable, and Lines of Credit Real Estate Loans Narrative (Details)					3 Months Ended		9 Months Ended		15 Months Ended
	Dec. 17, 2019 USD ($)	Dec. 10, 2019 USD ($)	Mar. 28, 2019 USD ($) loan	May 23, 2018 USD ($) loan	Sep. 30, 2020 USD ($) number_of_properties	Jun. 30, 2020 USD ($)	Sep. 30, 2020 USD ($) number_of_properties	Sep. 30, 2019 USD ($)	Jun. 30, 2020 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2020	Sep. 30, 2020 ft²	Sep. 03, 2020 USD ($) number_of_properties	May 14, 2020 USD ($) number_of_properties	Feb. 28, 2020 USD ($) number_of_properties	Dec. 31, 2019 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)
Mortgage Loans on Real Estate [Line Items]
Payments to Acquire Investments							$ 50,000	$ 0
payments received from real estate loan participants							$ 0	5,223,000
Variable Interest Entity, Reporting Entity Involvement, Maximum Loss Exposure, Amount										$ 323,600,000
variable interest entity loans amount to be funded										386,600,000
real estate loan balances unfunded										63,000,000.0
Number of units in real estate property											12,936	680				12,256
Real Estate Loan Investments Collateralized, Percent											100.00%
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest						$ 1,709,996,000		1,862,930,000	$ 1,709,996,000	1,670,858,000						$ 1,934,116,000	$ 1,784,170,000	$ 1,609,385,000
Real Estate Loan Investments, Credit Loss Expense					$ 800,000
Real Estate Loan Investment, Number Of Loans | number_of_properties					25		25
Real Estate Loan Investment										340,602,000
Proceeds from Sale of Notes Receivable	$ 20,400,000	$ 6,200,000
Real Estate Loan Investment, Reserve Ratio 0.00
Mortgage Loans on Real Estate [Line Items]
Real Estate Loan Investment, Number Of Loans | number_of_properties					3		3
Real Estate Loan Investment										28,921,000
Real Estate Loan Investment, Reserve Ratio 0.50
Mortgage Loans on Real Estate [Line Items]
Real Estate Loan Investment, Number Of Loans | number_of_properties					10		10
Real Estate Loan Investment										108,630,000
Real Estate Loan Investment, Reserve Ratio 1.00
Mortgage Loans on Real Estate [Line Items]
Real Estate Loan Investment, Number Of Loans | number_of_properties					3		3
Real Estate Loan Investment										17,876,000
Real Estate Loan Investment, Reserve Ratio 1.50
Mortgage Loans on Real Estate [Line Items]
Real Estate Loan Investment, Number Of Loans | number_of_properties					4		4
Real Estate Loan Investment										26,729,000
Real Estate Loan Investment, Reserve Ratio 3.00
Mortgage Loans on Real Estate [Line Items]
Real Estate Loan Investment, Number Of Loans | number_of_properties					3		3
Real Estate Loan Investment										27,902,000
Real Estate Loan Investment, Reserve Ratio 4.00
Mortgage Loans on Real Estate [Line Items]
Real Estate Loan Investment, Number Of Loans | number_of_properties					1		1
Real Estate Loan Investment										126,768,000
Real Estate Loan Investment, Reserve Ratio 5.00 And Above
Mortgage Loans on Real Estate [Line Items]
Real Estate Loan Investment, Number Of Loans | number_of_properties					1		1
Real Estate Loan Investment										3,776,000
Accumulated Deficit [Member]
Mortgage Loans on Real Estate [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest						(206,724,000)		$ 0	(206,724,000)	(210,218,000)						(7,244,000)	$ 0	$ 0
Accounting Standards Update 2016-13
Mortgage Loans on Real Estate [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest																(7,414,000)
Accounting Standards Update 2016-13 | Accumulated Deficit [Member]
Mortgage Loans on Real Estate [Line Items]
Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest																(7,414,000)
Berryessa [Member]
Mortgage Loans on Real Estate [Line Items]
Real Estate Loan Investment, Reserve Ratio											4.00%
Starkville [Member]
Mortgage Loans on Real Estate [Line Items]
Real Estate Loan Investments, Credit Loss Expense					$ 0	$ 2,100,000
Real Estate Loan Investment										3,800,000						5,900,000
Allowance for Loan and Lease Losses, Real Estate																1,400,000
Real Estate Loan Investment, Initial Amount																7,300,000
Real Estate Loan Investment, Accrued Interest																$ 1,200,000
Multifamily
Mortgage Loans on Real Estate [Line Items]
Number of units in real estate property | number_of_properties					1		1
Multifamily | Multifamily Community In Raleigh, Atlanta Georgia [Member]
Mortgage Loans on Real Estate [Line Items]
Real Estate Loan Investment, Maximum													$ 20,700,000
Number of units in real estate property | number_of_properties													320
Loan Receivable, Fixed Interest Rate													8.50%
Loan Receivable, Deferred Fixed Interest														5.50%
Multifamily | Multifamily Community In Raleigh, North Carolina [Member]
Mortgage Loans on Real Estate [Line Items]
Real Estate Loan Investment, Maximum														$ 10,000,000.0
Number of units in real estate property | number_of_properties														277
Loan Receivable, Fixed Interest Rate														8.50%
Loan Receivable, Deferred Fixed Interest														5.50%
Multifamily | Multifamily Community In Charlotte, North Carolina [Member]
Mortgage Loans on Real Estate [Line Items]
Real Estate Loan Investment, Maximum															$ 13,400,000
Number of units in real estate property | number_of_properties															256
Loan Receivable, Fixed Interest Rate															8.50%
Loan Receivable, Deferred Fixed Interest															5.50%
Mortgage Backed Securities, Other [Member]
Mortgage Loans on Real Estate [Line Items]
Monthly Interest Expense			$ 103,000
Payments to Acquire Investments			$ 18,400,000	$ 4,700,000
number of loans in CMBS trust | loan			21	20
total maturity amount of CMBS pool			$ 295,700,000	$ 276,300,000
HCC [Member]
Mortgage Loans on Real Estate [Line Items]
interest rate note receivable											8.00%
Loan Receivable Default Interest Rate											10.00%
Loan Receivable, Default Interest Income										1,800,000
Proceeds from Collection of Notes Receivable									$ 3,750,000
Geographic Concentration Risk [Member] | GEORGIA
Mortgage Loans on Real Estate [Line Items]
amount drawn under loan agreement										56,500,000
loan commitment amount										86,200,000
Geographic Concentration Risk [Member] | FLORIDA
Mortgage Loans on Real Estate [Line Items]
amount drawn under loan agreement										56,400,000
loan commitment amount										$ 61,200,000