Consolidated K-Series - Narrative (Details) $ in Millions	Sep. 30, 2019 USD ($) securitization	Dec. 31, 2018 USD ($) securitization
Multi-family loans held in securitization trusts
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 14,700.0	$ 11,500.0
Multi-family collateralized debt obligations, at fair value
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Carrying Value	$ 14,700.0	$ 11,500.0
Weighted average interest rate	4.11%	3.96%
Consolidated K-Series
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
K-Series, carrying value	$ 885.1	$ 657.6
Consolidated K-Series | Freddie Mac Multi-family loan K-Series
SEC Schedule, 12-29, Real Estate Companies, Investment in Mortgage Loans on Real Estate [Line Items]
Number of securitizations | securitization	12	9