Summary of Certain Characteristics of Derivatives (Detail) - Jun. 30, 2015 - USD ($) $ in Thousands	Total
Swap equivalent Eurodollar contracts \| 2-year
Derivative Instruments:
Derivative Instruments, maturity period	2 years
Average Years to Maturity	2 years
Us Treasury futures \| 5 year
Derivative Instruments:
Derivative Instruments, maturity period	5 years
Average Years to Maturity	4 years 5 months 1 day
Us Treasury futures \| 10 year and greater
Derivative Instruments:
Derivative Instruments minimum maturity period	10 years
Average Years to Maturity	6 years 11 months 1 day
Future
Derivative Instruments:
Average Years to Maturity	3 years 3 months 4 days
Short \| Swap equivalent Eurodollar contracts \| 2-year
Derivative Instruments:
Notional - Long Positions	$ (5,000,000)
Short \| Us Treasury futures \| 5 year
Derivative Instruments:
Notional - Long Positions	(2,273,000)
Short \| Us Treasury futures \| 10 year and greater
Derivative Instruments:
Notional - Long Positions	(1,007,500)
Short \| Future
Derivative Instruments:
Notional - Long Positions	$ (8,280,500)

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 Instruments Minimum Maturity Period

+ References

No definition available.

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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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- 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.

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