BORROWINGS - Types of Repurchase Agreements (Details) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Short-term Debt [Line Items]
Amount outstanding	$ 1,234,522,000	$ 804,811,000
Repurchase agreements, weighted average interest rate (as a percentage)	1.56%	1.07%
Market value of collateral held	$ 1,295,225,428	$ 876,121,505
Agency
Short-term Debt [Line Items]
Amount outstanding	$ 1,228,349,000	$ 755,221,000
Repurchase agreements, weighted average interest rate (as a percentage)	1.55%	0.97%
Market value of collateral held	$ 1,285,083,649	$ 790,190,232
Non-Agency
Short-term Debt [Line Items]
Amount outstanding	$ 2,555,000	$ 7,313,000
Repurchase agreements, weighted average interest rate (as a percentage)	3.38%	2.39%
Market value of collateral held	$ 4,399,779	$ 12,784,707
Multi-Family
Short-term Debt [Line Items]
Amount outstanding	$ 3,618,000	$ 42,277,000
Repurchase agreements, weighted average interest rate (as a percentage)	3.16%	2.52%
Market value of collateral held	$ 5,742,000	$ 73,146,566

This is the weighted average interest rate during the reporting period as a result of repurchase agreements.

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- Definition

Fair value of collateral permitted, by contact or custom, to be sold or re-pledged.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=64858031&loc=SL6224234-111729

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- Definition

Carrying value, as of the balance sheet date, of securities sold under agreements to repurchase where the transferor maintains effective control over the assets, accounting for them as secured debt.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(m)(1)(i))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 10
-Section 55
-Paragraph 51
-URI http://asc.fasb.org/extlink&oid=77903054&loc=d3e101761-111710

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+ References

No definition available.

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