Summary of Significant Accounting Policies - Real Estate Loans Receivable (Details)	Sep. 30, 2019 mezzanine_loan
Mezzanine Loan Receivable
Property, Plant and Equipment [Line Items]
Number of mezzanine loans collateralized	12
Construction Loans
Property, Plant and Equipment [Line Items]
Number of mezzanine loans collateralized	2
Term Loan Receivable
Property, Plant and Equipment [Line Items]
Number of mezzanine loans collateralized	2