Agency Mortgage-Backed Securities - Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2014	Dec. 31, 2013
Mortgage-Backed Securities Portfolio:
Estimated fair value	$ 81,565,256	$ 70,388,949
Agency Mortgage Backed Securities
Mortgage-Backed Securities Portfolio:
Principal outstanding	75,982,999	68,144,321
Unamortized premium	5,368,726	4,625,864
Unamortized discount	(18,200)	(21,484)
Amortized cost	81,333,525	72,748,701
Gross unrealized gains	982,001	693,325
Gross unrealized losses	(750,270)	(3,053,077)
Estimated fair value	81,565,256	70,388,949
Agency Mortgage Backed Securities \| Fixed Rate
Mortgage-Backed Securities Portfolio:
Amortized cost	78,250,313	68,784,424
Gross unrealized gains	847,615	538,556
Gross unrealized losses	(732,533)	(3,040,153)
Estimated fair value	78,365,395	66,282,827
Agency Mortgage Backed Securities \| Adjustable Rate
Mortgage-Backed Securities Portfolio:
Amortized cost	3,083,212	3,964,277
Gross unrealized gains	134,386	154,769
Gross unrealized losses	(17,737)	(12,924)
Estimated fair value	3,199,861	4,106,122
Agency Mortgage Backed Securities \| Freddie Mac
Mortgage-Backed Securities Portfolio:
Principal outstanding	27,906,221	24,458,925
Unamortized premium	1,951,798	1,627,966
Unamortized discount	(8,985)	(9,533)
Amortized cost	29,849,034	26,077,358
Gross unrealized gains	313,761	227,423
Gross unrealized losses	(322,094)	(1,267,106)
Estimated fair value	29,840,701	25,037,675
Agency Mortgage Backed Securities \| Fannie Mae
Mortgage-Backed Securities Portfolio:
Principal outstanding	47,979,778	43,564,657
Unamortized premium	3,396,368	2,970,813
Unamortized discount	(8,857)	(11,568)
Amortized cost	51,367,289	46,523,902
Gross unrealized gains	660,230	456,057
Gross unrealized losses	(424,800)	(1,781,683)
Estimated fair value	51,602,719	45,198,276
Agency Mortgage Backed Securities \| Ginnie Mae
Mortgage-Backed Securities Portfolio:
Principal outstanding	97,000	120,739
Unamortized premium	20,560	27,085
Unamortized discount	(358)	(383)
Amortized cost	117,202	147,441
Gross unrealized gains	8,010	9,845
Gross unrealized losses	(3,376)	(4,288)
Estimated fair value	$ 121,836	$ 152,998

The outstanding principal amount of the debt security before any adjustments for unamortized premium or discount.

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- Definition

The amount of discount that was originally recognized at the issuance of the debt security that has yet to be accreted.

+ References

No definition available.

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- Definition

The amount of premium that was originally recognized at the issuance of the debt security that has yet to be amortized.

+ References

No definition available.

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- Definition

Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (b)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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- Definition

Amount before tax of unrealized loss in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 320

-SubTopic 10

-Section 50

-Paragraph 2

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27161-111563

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- Definition

This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.2)

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

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- Definition

The fair value as of the balance sheet date of firm holdings in mortgages, mortgage backed securities (investment instruments that represent ownership of an undivided interest in a group of mortgages, where principal and interest from the individual mortgages are used to pay investors' principal and interest on the mortgage backed security) and asset-backed securities that are primarily paid from the cash flows of a discrete pool of receivables or other financial assets (such as credit-card receivables), either fixed or revolving, that by their terms convert into cash within a finite time period. Includes both pledged (to counterparties as collateral for financing transactions) and unpledged holdings.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 940

-SubTopic 320

-Section 45

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=35711157&loc=d3e42546-110969

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