FAIR VALUE OF FINANCIAL INSTRUMENTS (Changes in Investments that Use Level 3 Inputs) (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
FAIR VALUE OF FINANCIAL INSTRUMENTS [Abstract]
Balance as of beginning of period			$ 549,684	$ 457,947
Realized gains			0
Unrealized losses	$ 4,540	$ 7,974	(5,630)	28,847
Fundings of principal and change in unamortized origination fees			26,354	126,100
Repayments of loans			(13,878)	(361)
Payment-in-kind interest			15,219	22,069
Reclassification of self-storage real estate owned			(255,495)	(125,715)
Net transfers in or out of Level 3			0	0
Balance at end of period	$ 316,254	$ 508,887	$ 316,254	$ 508,887

Amount of decrease in investment in mortgage loan on real estate by entity with substantial portion of business acquiring and holding investment real estate or interest in real estate, from collection of principal.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 12
-Footnote 6
-Subsection 29

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 948
-SubTopic 310
-Section S99
-Paragraph 1
-Subparagraph (SX 210.12-29(Footnote 6))
-URI http://asc.fasb.org/extlink&oid=6589523&loc=d3e617274-123014

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Amount of issuances of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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-SubTopic 10
-Section 50
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Amount of transfer of financial instrument classified as an asset into (out of) level 3 of the fair value hierarchy.

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Fair value of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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Interest paid other than in cash for example by issuing additional debt securities. As a noncash item, it is added to net income when calculating cash provided by or used in operations using the indirect method.

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-Topic 230
-SubTopic 10
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The net realized gains or losses on investments during the period, not including gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity.

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Value transferred from mortgage loans to real estate owned (REO) in noncash transactions.

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The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of investments, not including unrealized gains or losses on securities separately or otherwise categorized as trading, available-for-sale, or held-to-maturity, held at each balance sheet date and included in earnings for the period.

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