LOANS - Investment Loan Activity (Details) $ in Thousands	9 Months Ended
	Sep. 30, 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],[2]
Impact of adopting CECL	(33,253)
Purchases / originations	1,808,766
Sales and transfers	(2,728,213)
Principal payments	(621,804)
Gains / (losses)	(106,481)
(Amortization) / accretion	6,976
Ending balance	2,788,341	[2]
Residential
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	1,647,787
Impact of adopting CECL	0
Purchases / originations	935,232
Sales and transfers	(2,258,330)
Principal payments	(151,708)
Gains / (losses)	(14,980)
(Amortization) / accretion	(5,042)
Ending balance	152,959
Commercial
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	669,713
Impact of adopting CECL	(3,600)
Purchases / originations	194,926
Sales and transfers	(148,036)
Principal payments	(59,675)
Gains / (losses)	(81,659)
(Amortization) / accretion	1,835
Ending balance	573,504
Corporate Debt
SEC Schedule, 12-29, Real Estate Companies, Investment in Movement in Mortgage Loans on Real Estate [Roll Forward]
Beginning balance	2,144,850
Impact of adopting CECL	(29,653)
Purchases / originations	678,608
Sales and transfers	(321,847)
Principal payments	(410,421)
Gains / (losses)	(9,842)
(Amortization) / accretion	10,183
Ending balance	$ 2,061,878

[1] Derived from the audited consolidated financial statements at December 31, 2019.

[2] Includes $50.0 million and $66.7 million of residential mortgage loans held for sale at September 30, 2020 and December 31, 2019, respectively.