SUBSEQUENT EVENTS (Details) - USD ($) $ / shares in Units, $ in Thousands	May 03, 2022	Apr. 21, 2022	Apr. 15, 2022	Mar. 31, 2022	Dec. 31, 2021
Subsequent Event [Line Items]
Cash dividends payable (in dollars per share)				$ 0.33
Supplemental cash dividend payable (in dollars per share)				$ 0.02
Outstanding principal				$ 2,439,373	$ 2,429,112
Loans held for investment ($1,028,398 and $974,424 related to consolidated VIEs, respectively)				$ 2,421,772	$ 2,414,383
Subsequent Event
Subsequent Event [Line Items]
Cash dividends payable (in dollars per share)	$ 0.33
Supplemental cash dividend payable (in dollars per share)	$ 0.02
Subsequent Event | Office | Senior Mortgage Loans | MASSACHUSETTS | SOFR Plus 3.75%
Subsequent Event [Line Items]
Outstanding principal			$ 82,200
Loans held for investment ($1,028,398 and $974,424 related to consolidated VIEs, respectively)			$ 19,300
Subsequent Event | Office | Senior Mortgage Loans | MASSACHUSETTS | SOFR Plus 3.75% | SOFR
Subsequent Event [Line Items]
Basis spread on variable rate			3.75%
Subsequent Event | Self Storage | Senior Mortgage Loans | MASSACHUSETTS | Libor Plus 2.90%
Subsequent Event [Line Items]
Outstanding principal		$ 6,800
Loans held for investment ($1,028,398 and $974,424 related to consolidated VIEs, respectively)		$ 6,300
Subsequent Event | Self Storage | Senior Mortgage Loans | MASSACHUSETTS | Libor Plus 2.90% | LIBOR
Subsequent Event [Line Items]
Basis spread on variable rate		2.90%
Subsequent Event | Self Storage | Senior Mortgage Loans | MASSACHUSETTS | Libor Plus 2 Point 90 Percent, Instrument 2
Subsequent Event [Line Items]
Outstanding principal		$ 7,700
Subsequent Event | Self Storage | Senior Mortgage Loans | MASSACHUSETTS | Libor Plus 2 Point 90 Percent, Instrument 2 | LIBOR
Subsequent Event [Line Items]
Basis spread on variable rate		2.90%
Subsequent Event | Self Storage | Senior Mortgage Loans | FLORIDA | Libor Plus 2.90%
Subsequent Event [Line Items]
Outstanding principal		$ 4,500
Subsequent Event | Self Storage | Senior Mortgage Loans | FLORIDA | Libor Plus 2.90% | LIBOR
Subsequent Event [Line Items]
Basis spread on variable rate		2.90%
Subsequent Event | Self Storage | Senior Mortgage Loans | PENNSYLVANIA | Libor Plus 3.05%
Subsequent Event [Line Items]
Outstanding principal		$ 13,800
Subsequent Event | Self Storage | Senior Mortgage Loans | PENNSYLVANIA | Libor Plus 3.05% | LIBOR
Subsequent Event [Line Items]
Basis spread on variable rate		3.05%
Subsequent Event | Self Storage | Senior Mortgage Loans | TEXAS | Libor Plus 2.90%
Subsequent Event [Line Items]
Outstanding principal		$ 8,000
Subsequent Event | Self Storage | Senior Mortgage Loans | TEXAS | Libor Plus 2.90% | LIBOR
Subsequent Event [Line Items]
Basis spread on variable rate		2.90%