Agency Mortgage-Backed Securities - Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2012	Dec. 31, 2011
Mortgage-Backed Securities Portfolio:
Estimated fair value	$ 123,963,207	$ 104,251,055
Agency Mortgage Backed Securities [Member]
Mortgage-Backed Securities Portfolio:
Agency mortgage-backed securities, par value	115,220,004	97,962,882
Unamortized discount	(22,219)	(23,905)
Unamortized premium	5,856,207	3,360,968
Amortized cost	121,053,992	101,299,945
Gross unrealized gain	3,097,216	3,086,232
Gross unrealized loss	(188,001)	(135,122)
Estimated fair value	123,963,207	104,251,055
Agency Mortgage Backed Securities [Member] \| Freddie Mac [Member]
Mortgage-Backed Securities Portfolio:
Agency mortgage-backed securities, par value	44,296,234	34,395,542
Unamortized discount	(9,515)	(9,874)
Unamortized premium	2,121,478	1,139,881
Amortized cost	46,408,197	35,525,549
Gross unrealized gain	1,166,299	973,476
Gross unrealized loss	(36,890)	(15,243)
Estimated fair value	47,537,606	36,483,782
Agency Mortgage Backed Securities [Member] \| Fannie Mae [Member]
Mortgage-Backed Securities Portfolio:
Agency mortgage-backed securities, par value	70,649,782	63,066,372
Unamortized discount	(12,315)	(13,632)
Unamortized premium	3,695,381	2,205,138
Amortized cost	74,332,848	65,257,878
Gross unrealized gain	1,913,334	2,081,282
Gross unrealized loss	(146,533)	(118,871)
Estimated fair value	76,099,649	67,220,289
Agency Mortgage Backed Securities [Member] \| Ginnie Mae [Member]
Mortgage-Backed Securities Portfolio:
Agency mortgage-backed securities, par value	273,988	500,968
Unamortized discount	(389)	(399)
Unamortized premium	39,348	15,949
Amortized cost	312,947	516,518
Gross unrealized gain	17,583	31,474
Gross unrealized loss	(4,578)	(1,008)
Estimated fair value	325,952	546,984
Agency Mortgage Backed Securities [Member] \| Adjustable Rate [Member]
Mortgage-Backed Securities Portfolio:
Amortized cost	5,786,718	8,698,746
Gross unrealized gain	259,013	345,642
Gross unrealized loss	(4,613)	(3,188)
Estimated fair value	6,041,118	9,041,200
Agency Mortgage Backed Securities [Member] \| Fixed Rate [Member]
Mortgage-Backed Securities Portfolio:
Amortized cost	115,267,274	92,601,199
Gross unrealized gain	2,838,203	2,740,590
Gross unrealized loss	(183,388)	(131,934)
Estimated fair value	$ 117,922,089	$ 95,209,855

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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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)

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-Publisher FASB

-Name FASB Staff Position (FSP)

-Number FAS115-1/124-1

-Paragraph 7

-Footnote 2

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 19

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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This item represents the gross unrealized gains for securities, at a point in time, which are categorized neither as held-to-maturity nor trading securities.

+ References

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

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 133

-Paragraph 22

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

Reference 2: 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=6872113&loc=d3e27161-111563

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Statement of Financial Accounting Standard (FAS)

-Number 115

-Paragraph 13, 19

-LegacyDoc This reference is SUPERSEDED by the Accounting Standards Codification effective for interim and annual periods ending after September 15, 2009. This reference is included to help users transition from the previous accounting hierarchy and will be removed from future versions of this taxonomy.

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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=6475986&loc=d3e42546-110969

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