PLEDGED ASSETS - Assets Pledged to Counterparties (Details) - USD ($)	Sep. 30, 2018	Dec. 31, 2017
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value	$ 3,498,724,000	$ 3,712,561,000
Accrued interest receivable	14,085,000	14,444,000
Restricted cash	30,928,000	32,349,000
Repurchase Agreement [Member]
Pledged Assets [Line Items]
Accrued interest receivable	13,740,000	13,962,000
Restricted cash	25,816,000	25,296,000
Derivative [Member]
Pledged Assets [Line Items]
Accrued interest receivable	36,000	0
Restricted cash	5,112,000	7,053,000
Residential Mortgage Backed Securities [Member] \| Repurchase Agreement [Member] \| Pass Through Certificate [Member]
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value	3,366,607,000	3,612,244,000
Residential Mortgage Backed Securities [Member] \| Repurchase Agreement [Member] \| Structured Finance [Member]
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value	121,163,000	$ 100,317,000
Residential Mortgage Backed Securities [Member] \| Derivative [Member] \| Pass Through Certificate [Member]
Pledged Assets [Line Items]
Financial Instruments, Owned and Pledged as Collateral, at Fair Value	$ 10,955,000

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- Definition

The fair value as of the balance sheet date of financial instruments and other positions owned by the entity and pledged to counterparties as collateral, including: (1) mortgages, mortgage-backed and asset backed securities; (2) US government and agency obligations; (3) state and municipal government obligations; (4) other sovereign government debt; (5) corporate obligations; (6) corporate equities; (7) principal investments; (8) derivative contracts; and (9) physical commodities.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=66007379&loc=d3e113888-111728

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-Publisher FASB
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-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

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- Definition

Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

Amount of cash and cash equivalents restricted as to withdrawal or usage, classified as current. Cash includes, but is not limited to, currency on hand, demand deposits with banks or financial institutions, and other accounts with general characteristics of demand deposits. Cash equivalents include, but are not limited to, short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section 45
-Paragraph 1
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-Publisher FASB
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-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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