FAIR VALUE - Financial Instruments Other than Debt Securities (Details) - USD ($) $ in Thousands	Mar. 31, 2020	Dec. 31, 2019
Financial assets [Abstract]
Residential mortgage loans	$ 8,503,171	$ 11,222,182
Derivatives at fair value		1,471
Financial liabilities [Abstract]
Derivatives at fair value	50,862	29,156
Carrying Amount [Member] \| Level 2 [Member]
Financial assets [Abstract]
Residential mortgage loans	591	637
Financial liabilities [Abstract]
Unsecured borrowings	98,418	98,392
Fair Value [Member] \| Level 2 [Member]
Financial assets [Abstract]
Residential mortgage loans	600	650
Financial liabilities [Abstract]
Unsecured borrowings	14,800	68,100
Interest Rate Swap Agreements [Member] \| Carrying Amount [Member] \| Secured Borrowings [Member] \| Level 2 [Member]
Financial assets [Abstract]
Secured borrowings-related interest rate swap agreements		733
Interest Rate Swap Agreements [Member] \| Carrying Amount [Member] \| Unsecured Borrowings [Member] \| Level 2 [Member]
Financial liabilities [Abstract]
Interest rate swap agreements	49,824	29,156
Interest Rate Swap Agreements [Member] \| Fair Value [Member] \| Secured Borrowings [Member] \| Level 2 [Member]
Financial assets [Abstract]
Secured borrowings-related interest rate swap agreements		733
Interest Rate Swap Agreements [Member] \| Fair Value [Member] \| Unsecured Borrowings [Member] \| Level 2 [Member]
Financial liabilities [Abstract]
Interest rate swap agreements	49,824	29,156
Eurodollar Futures Contract [Member] \| Carrying Amount [Member] \| Level 1 [Member]
Financial assets [Abstract]
Derivatives at fair value		738
Financial liabilities [Abstract]
Derivatives at fair value	1,038
Eurodollar Futures Contract [Member] \| Fair Value [Member] \| Level 1 [Member]
Financial assets [Abstract]
Derivatives at fair value		$ 738
Financial liabilities [Abstract]
Derivatives at fair value	$ 1,038

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

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The fair value as of the balance sheet date of firm holdings in mortgages, mortgage backed securities (investment instruments that represent ownership of an undivided interest in a group of mortgages, where principal and interest from the individual mortgages are used to pay investors' principal and interest on the mortgage backed security) and asset-backed securities that are primarily paid from the cash flows of a discrete pool of receivables or other financial assets (such as credit-card receivables), either fixed or revolving, that by their terms convert into cash within a finite time period. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

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The fair value amount of long-term debt whether such amount is presented as a separate caption or as a parenthetical disclosure. Additionally, this element may be used in connection with the fair value disclosures required in the footnote disclosures to the financial statements. The element may be used in both the balance sheet and disclosure in the same submission.

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