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DERIVATIVE INSTRUMENTS - Summary of Certain Characteristics of Futures Derivatives (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2018
Swap equivalent Eurodollar futures contract | 2 Year
Derivative [Line Items]
Derivative Instruments, maturity period	2 years	2 years
Weighted Average Underlying Years to Maturity		1 year 11 months 20 days	2 years
Swap equivalent Eurodollar futures contract | 2 Year | Long
Derivative [Line Items]
Notional		$ 0		$ 0
Swap equivalent Eurodollar futures contract | 2 Year | Short
Derivative [Line Items]
Notional		$ (1,166,000)		$ (2,500,000)
U.S. Treasury futures | 2 Year
Derivative [Line Items]
Weighted Average Underlying Years to Maturity	1 year 11 months 6 days
U.S. Treasury futures | 2 Year | Long
Derivative [Line Items]
Notional	$ 0
U.S. Treasury futures | 2 Year | Short
Derivative [Line Items]
Notional	$ (2,872,400)
U.S. Treasury futures | 5 Year
Derivative [Line Items]
Derivative Instruments, maturity period	5 years	5 years
Weighted Average Underlying Years to Maturity	4 years 4 months 22 days	4 years 4 months 22 days
U.S. Treasury futures | 5 Year | Long
Derivative [Line Items]
Notional	$ 0	$ 0
U.S. Treasury futures | 5 Year | Short
Derivative [Line Items]
Notional	$ (6,469,400)	$ (6,359,400)
U.S. Treasury futures | 10 Year and Greater
Derivative [Line Items]
Derivative Instruments minimum maturity period	10 years	10 years
Weighted Average Underlying Years to Maturity	6 years 10 months 3 days	7 years 1 month 7 days
U.S. Treasury futures | 10 Year and Greater | Long
Derivative [Line Items]
Notional	$ (109,000)	$ 0
U.S. Treasury futures | 10 Year and Greater | Short
Derivative [Line Items]
Notional	$ (9,589,900)	$ (11,152,600)
Futures contracts
Derivative [Line Items]
Weighted Average Underlying Years to Maturity	4 years 10 months 19 days	5 years 10 months 10 days
Futures contracts | Long
Derivative [Line Items]
Notional	$ (109,000)	$ 0
Futures contracts | Short
Derivative [Line Items]
Notional	$ (21,431,700)	$ (18,678,000)

X

- Definition

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

+ References

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

+ Details

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X

- Definition

Derivative instruments minimum maturity period.

+ References

No definition available.

+ Details

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X

- 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.

+ Details

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X

- 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.

+ Details

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X

- Definition

Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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X

- Details

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- Details

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