FAIR VALUE - Financial Instruments Other than Debt Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2020	Dec. 31, 2019
Financial assets [Abstract]
Residential mortgage loans	$ 8,274,509	$ 11,222,182
Derivatives at fair value		1,471
Carrying Amount [Member] \| Level 2 [Member]
Financial assets [Abstract]
Residential mortgage loans	487	637
Financial liabilities [Abstract]
Unsecured borrowings	98,468	98,392
Secured borrowings with initial terms of greater than 120 days	500,000
Fair Value [Member] \| Level 2 [Member]
Financial assets [Abstract]
Residential mortgage loans	500	650
Financial liabilities [Abstract]
Unsecured borrowings	44,500	68,100
Secured borrowings with initial terms of greater than 120 days	500,050
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	46,529	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	$ 46,529	29,156
Eurodollar Futures Contract [Member] \| Carrying Amount [Member] \| Level 1 [Member]
Financial assets [Abstract]
Derivatives at fair value		738
Eurodollar Futures Contract [Member] \| Fair Value [Member] \| Level 1 [Member]
Financial assets [Abstract]
Derivatives at fair value		$ 738

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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Fair value, after the effects of master netting arrangements, of a financial liability or 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 liabilities not subject to a master netting arrangement and not elected to be offset.

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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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Carrying value as of the balance sheet date, including the current and noncurrent portions, of collateralized debt obligations (with maturities initially due after one year or beyond the operating cycle, if longer). Such obligations include mortgage loans, chattel loans, and any other borrowings secured by assets of the borrower.

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