Schedule of Agency Securities Forward Purchase Commitments (Parenthetical) (Detail)	9 Months Ended
Sep. 30, 2012 D
Schedule of Investment Income, Reported Amounts, by Category [Line Items]
Days to coupon reset or contractual payment, minimum range	30
Days to coupon reset or contractual payment, maximum range	90

Days To Coupon Reset Or Contractual Payment Maximum Range

No definition available.

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Period Type:	duration