Agency Mortgage-Backed Securities - Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2013	Dec. 31, 2012
Mortgage-Backed Securities Portfolio:
Estimated fair value	$ 70,388,949	$ 123,963,207
Agency Mortgage Backed Securities
Mortgage-Backed Securities Portfolio:
Principal outstanding	68,144,321	115,220,004
Unamortized premium	4,625,864	5,856,207
Unamortized discount	(21,484)	(22,219)
Amortized cost	72,748,701	121,053,992
Gross unrealized gains	693,325	3,097,216
Gross unrealized losses	(3,053,077)	(188,001)
Estimated fair value	70,388,949	123,963,207
Agency Mortgage Backed Securities \| Fixed Rate
Mortgage-Backed Securities Portfolio:
Amortized cost	68,784,424	115,267,274
Gross unrealized gains	538,556	2,838,203
Gross unrealized losses	(3,040,153)	(183,388)
Estimated fair value	66,282,827	117,922,089
Agency Mortgage Backed Securities \| Adjustable Rate
Mortgage-Backed Securities Portfolio:
Amortized cost	3,964,277	5,786,718
Gross unrealized gains	154,769	259,013
Gross unrealized losses	(12,924)	(4,613)
Estimated fair value	4,106,122	6,041,118
Agency Mortgage Backed Securities \| Freddie Mac
Mortgage-Backed Securities Portfolio:
Principal outstanding	24,458,925	44,296,234
Unamortized premium	1,627,966	2,121,478
Unamortized discount	(9,533)	(9,515)
Amortized cost	26,077,358	46,408,197
Gross unrealized gains	227,423	1,166,299
Gross unrealized losses	(1,267,106)	(36,890)
Estimated fair value	25,037,675	47,537,606
Agency Mortgage Backed Securities \| Fannie Mae
Mortgage-Backed Securities Portfolio:
Principal outstanding	43,564,657	70,649,782
Unamortized premium	2,970,813	3,695,381
Unamortized discount	(11,568)	(12,315)
Amortized cost	46,523,902	74,332,848
Gross unrealized gains	456,057	1,913,334
Gross unrealized losses	(1,781,683)	(146,533)
Estimated fair value	45,198,276	76,099,649
Agency Mortgage Backed Securities \| Ginnie Mae
Mortgage-Backed Securities Portfolio:
Principal outstanding	120,739	273,988
Unamortized premium	27,085	39,348
Unamortized discount	(383)	(389)
Amortized cost	147,441	312,947
Gross unrealized gains	9,845	17,583
Gross unrealized losses	(4,288)	(4,578)
Estimated fair value	$ 152,998	$ 325,952

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

No definition available.

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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 of accumulated pre-tax unrealized gain before deducting pre-tax unrealized losses on investments in available-for-sale securities impacting investments.

+ References

No definition available.

+ Details

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- Definition

Amount of accumulated pre-tax unrealized loss before deducting pre-tax unrealized gain on investments in available-for-sale securities impacting investments.

+ References

No definition available.

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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=29643736&loc=d3e42546-110969

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