Commercial Loans - Components of the carrying value of commercial real estate loans (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
CMSC Trust
Variable Interest Entity [Line Items]
Principal balance	$ 23,943	$ 24,048
Unamortized premium	0	0
Unamortized discount	0	0
Amortized cost	23,943	24,048
Gross unrealized gains	0	9
Gross unrealized losses	(147)	0
Fair value	23,796	24,057
RETL Trust
Variable Interest Entity [Line Items]
Principal balance	519,735	674,331
Unamortized premium	1,189	1,836
Unamortized discount	0	0
Amortized cost	520,924	676,167
Gross unrealized gains	0	269
Gross unrealized losses	(67,589)	0
Fair value	453,335	676,436
RSBC Trust
Variable Interest Entity [Line Items]
Principal balance	73,150	90,788
Unamortized premium	0	0
Unamortized discount	(110)	(215)
Amortized cost	73,040	90,573
Gross unrealized gains	0	215
Gross unrealized losses	(1,356)	0
Fair value	71,684	90,788
Commercial Mezzanine Loan
Variable Interest Entity [Line Items]
Principal balance	259,426	279,425
Unamortized premium	0	0
Unamortized discount	(203)	(294)
Amortized cost	259,223	279,131
Gross unrealized gains	0	294
Gross unrealized losses	(10,599)	0
Fair value	$ 248,624	$ 279,425