INVESTMENTS IN RESIDENTIAL MORTGAGE LOANS - Narrative (Details) $ in Thousands			9 Months Ended			12 Months Ended
	Dec. 31, 2019 USD ($)		Sep. 30, 2020 USD ($) trust		Sep. 30, 2019 USD ($) trust	Dec. 31, 2019 USD ($)		Jan. 01, 2020 USD ($)
Accounts, Notes, Loans and Financing Receivable [Line Items]
Accounting Standards Update [Extensible List]	us-gaap:AccountingStandardsUpdate201613Member		us-gaap:AccountingStandardsUpdate201613Member			us-gaap:AccountingStandardsUpdate201613Member
Residential mortgage loans subject to repurchase	$ 172,336	[1]	$ 1,458,325	[1]	$ 168,532	$ 172,336	[1]
Reduction to retained earnings	(549,733)		$ 1,094,589			(549,733)
Threshold period past due (in days)			60 days
Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Number of trusts called | trust			15		97
Residential Mortgage Loans | Loans Sold
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income			$ 12,000		$ 45,300
Gain on securitization accounted for as sales			$ 48,300		$ 54,200
Cumulative Effect, Period of Adoption, Adjustment
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reduction to retained earnings	$ (13,700)					$ (13,700)
Cumulative Effect, Period of Adoption, Adjustment | Residential Mortgage Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Reduction to retained earnings								$ 6,000

[1] See Note 8 for details.