Agency Mortgage-Backed Securities - Unrealized Loss Position (Detail) $ in Thousands	Sep. 30, 2015 USD ($) Securities	Dec. 31, 2014 USD ($) Securities
Unrealized Loss Position For:
Estimated Fair Value	$ 29,491,742	$ 39,788,793
Gross Unrealized Losses	$ (568,628)	$ (750,270)
Number of Securities \| Securities	569	507
Available For Sale Securities, Continuous Unrealized Loss Positions, Less Than 12 Months
Unrealized Loss Position For:
Estimated Fair Value	$ 6,574,181	$ 4,613,599
Gross Unrealized Losses	$ (61,722)	$ (36,959)
Number of Securities \| Securities	283	205
Available For Sale Securities, Continuous Unrealized Loss Positions, Greater Than 12 Months
Unrealized Loss Position For:
Estimated Fair Value	$ 22,917,561	$ 35,175,194
Gross Unrealized Losses	$ (506,906)	$ (713,311)
Number of Securities \| Securities	286	302

Mortgage-Backed Securities in a continuous unrealized loss position [Line Items]

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- Definition

Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale in a continuous loss position.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27337-111563

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (a)(2)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

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- Definition

This item represents the aggregate fair value of investments in debt and equity securities in an unrealized loss position which are categorized neither as held-to-maturity nor trading securities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (a)(1)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

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- Definition

Number of investment positions in available-for-sale investments in a continuous unrealized loss position for which an other-than-temporary impairment (OTTI) has not been recognized in the income statement.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 6

-Subparagraph (b)(3)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27290-111563

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