Mortgage-Backed Securities (Schedule Of Interest Income) (Details) (USD $) In Thousands, unless otherwise specified	3 Months Ended	12 Months Ended
Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Mortgage-Backed Securities [Abstract]
Coupon interest on MBS									$ 518,048	$ 305,529	$ 301,707
Net premium amortization									(93,335)	(41,778)	(19,363)
Interest income on MBS, net	$ 114,465	$ 110,125	$ 113,188	$ 86,935	$ 67,668	$ 63,701	$ 63,441	$ 68,941	$ 424,713	$ 263,751	$ 282,344

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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