Schedule of Agency Securities Portfolio Backed by U.S. Government Agency or U.S. Government Sponsored Entity (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	$ 25,398,721	$ 23,419,908
Gross Unrealized Loss	(343,090)	(121)
Gross Unrealized Gain	200,412	499,464
Carrying value/estimated fair value	25,256,043	23,919,251
Federal National Mortgage Association Certificates and Obligations (FNMA)
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	16,170,421	14,824,558
Gross Unrealized Loss	(207,697)	(100)
Gross Unrealized Gain	153,385	339,076
Carrying value/estimated fair value	16,116,109	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	14,043,290	14,081,259
Gross Unrealized Loss	(147,577)	(100)
Gross Unrealized Gain	153,385	329,780
Carrying value/estimated fair value	14,049,098	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	2,127,131	743,299
Gross Unrealized Loss	(60,120)
Gross Unrealized Gain		9,296
Carrying value/estimated fair value	2,067,011	752,595
Mortgage-backed Securities, Issued by US Government Sponsored Enterprises
Schedule of Available-for-sale Securities [Line Items]
Agency Securities Amortized Cost	9,228,300	8,595,350
Gross Unrealized Loss	(135,393)	(21)
Gross Unrealized Gain	47,027	160,388
Carrying value/estimated fair value	9,139,934	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	8,513,715	7,850,630
Gross Unrealized Loss	(115,264)	(21)
Gross Unrealized Gain	47,027	149,114
Carrying value/estimated fair value	8,445,478	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	714,585	744,720
Gross Unrealized Loss	(20,129)
Gross Unrealized Gain		11,274
Carrying value/estimated fair value	$ 694,456	$ 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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