Schedule of Agency Securities Portfolio Backed by U.S. Government Agency or U.S. Government Sponsored Entity (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	$ 19,801,065	$ 23,419,908
Gross Unrealized Loss	(183,020)	(121)
Gross Unrealized Gain	225,785	499,464
Carrying value/estimated fair value	19,843,830	23,919,251
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	12,264,125	14,824,558
Gross Unrealized Loss	(95,244)	(100)
Gross Unrealized Gain	173,289	339,076
Carrying value/estimated fair value	12,342,170	15,163,534
Federal National Mortgage Association Certificates and Obligations (FNMA) \| Fannie Mae Certificates, ARMs
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	11,217,817	14,081,259
Gross Unrealized Loss	(84,364)	(100)
Gross Unrealized Gain	162,310	329,780
Carrying value/estimated fair value	11,295,763	14,410,939
Federal National Mortgage Association Certificates and Obligations (FNMA) \| Fannie Mae Certificates, Fixed Rate
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	1,046,308	743,299
Gross Unrealized Loss	(10,880)
Gross Unrealized Gain	10,979	9,296
Carrying value/estimated fair value	1,046,407	752,595
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	7,536,940	8,595,350
Gross Unrealized Loss	(87,776)	(21)
Gross Unrealized Gain	52,496	160,388
Carrying value/estimated fair value	7,501,660	8,755,717
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises \| Freddie Mac Certificates, ARMs
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	7,485,569	7,850,630
Gross Unrealized Loss	(87,776)	(21)
Gross Unrealized Gain	52,292	149,114
Carrying value/estimated fair value	7,450,085	7,999,723
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises \| Freddie Mac Certificates, Fixed Rate
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	51,371	744,720
Gross Unrealized Gain	204	11,274
Carrying value/estimated fair value	$ 51,575	$ 755,994

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

No definition available.

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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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