Fair Value of Financial Instruments - Assets Measured at Fair Value on a Nonrecurring Basis (Details) - Fair Value, Measurements, Nonrecurring - Residential mortgage loans held in securitization trusts – impaired loans, net - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential mortgage loans held in securitization trusts – impaired loans, net	$ 5,350	$ 5,921
Level 1
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential mortgage loans held in securitization trusts – impaired loans, net	0	0
Level 2
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential mortgage loans held in securitization trusts – impaired loans, net	0	0
Level 3
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Residential mortgage loans held in securitization trusts – impaired loans, net	$ 5,350	$ 5,921