DERIVATIVE INSTRUMENTS - Summary of Swaptions Outstanding (Details) - Interest rate swaptions - Long - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2018	Dec. 31, 2017
Derivative Instruments and Hedging Activities Disclosures [Line Items]
Current Notional	$ 6,000,000	$ 6,000,000
Weighted Average Underlying Pay Rate	2.72%	2.62%
Weighted Average Underlying Years to Maturity	10 years 4 months	9 years 11 months 19 days
Weighted Average Months to Expiration	3 months 27 days	4 months 15 days

Aggregate notional amount specified by the derivative(s). Expressed as an absolute value.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Article 12
-Section 13
-Sentence Column B

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- Definition

Derivative Portfolio Expiration Period

+ References

No definition available.

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- Definition

Interest rate swaptions weighted average underlying pay rate.

+ References

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- Definition

Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

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- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

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