LOANS - Investment Loan Activity (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020 USD ($)
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	$ 4,462,350	[1]
Purchases / originations	2,447,803
Sales and transfers	(2,891,854)
Principal payments	(809,045)
Gains / (losses)	(101,248)
(Amortization) / accretion	9,067
Ending balance	3,083,821	[1]
Transfer of residential loans to securitization vehicles, carrying value, noncash	1,900,000
Cumulative Effect, Period of Adoption, Adjustment
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	(33,252)
Ending balance
Residential
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	1,647,787
Purchases / originations	1,168,830
Sales and transfers	(2,298,391)
Principal payments	(154,864)
Gains / (losses)	(11,854)
(Amortization) / accretion	(5,698)
Ending balance	345,810
Residential | Cumulative Effect, Period of Adoption, Adjustment
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	0
Ending balance
Commercial
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	669,713
Purchases / originations	217,329
Sales and transfers	(235,533)
Principal payments	(77,422)
Gains / (losses)	(74,965)
(Amortization) / accretion	2,558
Ending balance	498,081
Commercial | Cumulative Effect, Period of Adoption, Adjustment
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	(3,599)
Ending balance
Corporate
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	2,144,850
Purchases / originations	1,061,644
Sales and transfers	(357,930)
Principal payments	(576,759)
Gains / (losses)	(14,429)
(Amortization) / accretion	12,207
Ending balance	2,239,930
Corporate | Cumulative Effect, Period of Adoption, Adjustment
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	$ (29,653)
Ending balance

[1] Includes $47.0 million and $66.7 million of residential mortgage loans held for sale.