Gross Gains and Losses from Investment Sales (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Gains (Losses) On Investments [Line Items]
Net gain (loss) on MBS	$ (225,635)	$ 10,534	$ (214,333)	$ 25,244
Mortgage Backed Securities
Gains (Losses) On Investments [Line Items]
Gross gains on MBS	7,327	10,534	18,629	25,244
Gross losses on MBS	$ (232,962)		$ (232,962)

Net gain (loss) resulting from the sales of securitized, pay-through debt securities collateralized by real estate mortgage loans (mortgages).

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 225

-Section S99

-Paragraph 1

-Subparagraph (SX 210.9-04.13(h))

-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration