Schedule Of Interest Income (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
Gain (Loss) on Investments [Line Items]
Coupon interest on MBS	$ 179,202	$ 140,234	$ 486,156	$ 370,967
Net premium amortization	(47,262)	(30,109)	(112,736)	(60,719)
Interest income on MBS, net	$ 131,940	$ 110,125	$ 373,420	$ 310,248