Agency Mortgage-Backed Securities - Portfolio (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Mortgage-Backed Securities Portfolio [Line Items]
Estimated fair value	$ 104,251,055	$ 78,440,330
Agency Mortgage Backed Securities [Member]
Mortgage-Backed Securities Portfolio [Line Items]
Agency mortgage-backed Securities, par value	97,962,882	75,011,712
Unamortized discount	(23,905)	(33,383)
Unamortized premium	3,360,968	2,338,823
Amortized cost	101,299,945	77,317,152
Gross unrealized gain	3,086,232	1,704,203
Gross unrealized loss	(135,122)	(581,025)
Estimated fair value	104,251,055	78,440,330
Agency Mortgage Backed Securities [Member] \| Adjustable Rate [Member]
Mortgage-Backed Securities Portfolio [Line Items]
Amortized cost	8,698,746	10,954,627
Gross unrealized gain	345,642	257,822
Gross unrealized loss	(3,188)	(75,440)
Estimated fair value	9,041,200	11,137,009
Agency Mortgage Backed Securities [Member] \| Fixed Rate [Member]
Mortgage-Backed Securities Portfolio [Line Items]
Amortized cost	92,601,199	66,362,525
Gross unrealized gain	2,740,590	1,446,381
Gross unrealized loss	(131,934)	(505,585)
Estimated fair value	95,209,855	67,303,321
Agency Mortgage Backed Securities [Member] \| Freddie Mac [Member]
Mortgage-Backed Securities Portfolio [Line Items]
Agency mortgage-backed Securities, par value	34,395,542	19,846,543
Unamortized discount	(9,874)	(14,651)
Unamortized premium	1,139,881	517,507
Amortized cost	35,525,549	20,349,399
Gross unrealized gain	973,476	463,471
Gross unrealized loss	(15,243)	(140,027)
Estimated fair value	36,483,782	20,672,843
Agency Mortgage Backed Securities [Member] \| Fannie Mae [Member]
Mortgage-Backed Securities Portfolio [Line Items]
Agency mortgage-backed Securities, par value	63,066,372	54,341,140
Unamortized discount	(13,632)	(18,329)
Unamortized premium	2,205,138	1,795,116
Amortized cost	65,257,878	56,117,927
Gross unrealized gain	2,081,282	1,211,324
Gross unrealized loss	(118,871)	(438,918)
Estimated fair value	67,220,289	56,890,333
Agency Mortgage Backed Securities [Member] \| Ginnie Mae [Member]
Mortgage-Backed Securities Portfolio [Line Items]
Agency mortgage-backed Securities, par value	500,968	824,029
Unamortized discount	(399)	(403)
Unamortized premium	15,949	26,200
Amortized cost	516,518	849,826
Gross unrealized gain	31,474	29,408
Gross unrealized loss	(1,008)	(2,080)
Estimated fair value	$ 546,984	$ 877,154

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

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 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 2: 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.

Reference 3: 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

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 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.

Reference 3: 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

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- Definition

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

+ References

No definition available.

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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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