Schedule Of Interest Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	6 Months Ended
Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Gain (Loss) on Investments [Line Items]
Coupon interest on MBS	$ 164,652	$ 130,943	$ 306,954	$ 230,733
Net premium amortization	(35,932)	(17,755)	(65,474)	(30,610)
Interest income on MBS, net	$ 128,720	$ 113,188	$ 241,480	$ 200,123

Represents interest income, including any amortization and accretion (as applicable) of discounts and premiums, derived from securities backed by a pool of mortgages or trust deeds. Mortgage-backed securities might represent mortgage pass-through securities, collateralized mortgage obligations (CMO), real estate mortgage investment conduits (REMICs), CMO and REMIC residuals, and stripped mortgage-backed securities.

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This item represents the amount of amortization of purchase premium related to an investment in debt securities. The purchase premium is amortized to expense over the life (holding period) of the security to arrive at an amount of periodic interest income which results in a constant effective yield on the investment.

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