Variable Interest Entities - Effect on Consolidated Results (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
Variable Interest Entity [Line Items]
Net interest income	$ 110,340	$ 152,789	$ 363,204	$ 433,321
Total other-than-temporary impairment losses	(2,713)	(78,950)	(47,264)	(161,924)
Portion of loss recognized in other comprehensive income (loss)	(7,301)	(71,610)	(76,801)	(83,989)
Net other-than-temporary credit impairment losses	(10,014)	(150,560)	(124,065)	(245,913)
Variable Interest Entity, Primary Beneficiary [Member]
Variable Interest Entity [Line Items]
Total other-than-temporary impairment losses	(241)	(36,082)	(7,510)	(62,970)
Portion of loss recognized in other comprehensive income (loss)	(3,032)	(100,701)	(76,164)	(112,985)
Net other-than-temporary credit impairment losses	(3,273)	(136,783)	(83,674)	(175,955)
Variable Interest Entity, Primary Beneficiary [Member] | Non Agency Residential Mortgage Backed Securities And Securitized Loans [Member]
Variable Interest Entity [Line Items]
Interest income, Assets of consolidated VIEs	108,120	116,805	315,962	339,765
Interest expense, Non-recourse liabilities of VIEs	(32,075)	(29,643)	(85,604)	(95,002)
Net interest income	$ 76,045	$ 87,162	$ 230,358	$ 244,763