LOANS - Investment Loan Activity (Details) $ in Thousands	3 Months Ended
	Mar. 31, 2021 USD ($)
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance January 1, 2021	$ 3,083,821	[1],[2]
Purchases / originations	653,267
Sales and transfers	(850,156)
Principal payments	(275,487)
Gains / (losses)	(11,177)
(Amortization) / accretion	3,075
Ending balance	2,603,343	[2]
Transfer of residential loans to securitization vehicles, carrying value, noncash	257,100
Residential
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance January 1, 2021	345,810
Purchases / originations	467,307
Sales and transfers	(267,551)
Principal payments	(10,281)
Gains / (losses)	(5,164)
(Amortization) / accretion	(1,253)
Ending balance	528,868
Commercial
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance January 1, 2021	498,081
Purchases / originations	114,467
Sales and transfers	(536,829)
Principal payments	(63,959)
Gains / (losses)	(12,199)
(Amortization) / accretion	439
Ending balance	0
Corporate Debt
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance January 1, 2021	2,239,930
Purchases / originations	71,493
Sales and transfers	(45,776)
Principal payments	(201,247)
Gains / (losses)	6,186
(Amortization) / accretion	3,889
Ending balance	$ 2,074,475

[1] Derived from the audited consolidated financial statements at December 31, 2020.

[2] Includes $46.0 million and $47.0 million of residential mortgage loans held for sale at March 31, 2021 and December 31, 2020, respectively.