Schedule Of Interest Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Gain (Loss) on Investments [Line Items]
Coupon interest on MBS	$ 163,964	$ 164,652	$ 331,397	$ 306,954
Net premium amortization	(48,849)	(35,932)	(92,041)	(65,474)
Interest income on MBS, net	$ 115,115	$ 128,720	$ 239,356	$ 241,480

Amount of operating interest income, including amortization and accretion of premiums and discounts, on securitized, pass-through debt securities collateralized by real estate mortgage loans.

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 210.9-04.2)

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

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 04

-Paragraph 2

-Article 9

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

-Publisher FASB

-Name Accounting Standards Codification

-Glossary Mortgage-Backed Securities

-URI http://asc.fasb.org/extlink&oid=6517959

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