Securitized Loans Held for Investment - Roll-Forward (Detail) (Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure [Member], Securitized Loans Held for Investment [Member], USD $) In Thousands, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Variable Interest Entities, Primary Beneficiary, Aggregated Disclosure [Member] | Securitized Loans Held for Investment [Member]
Securitized Loans Held For Investment By Consolidated Variable Interest Entities [Line Items]
Balance, beginning of year	$ 256,632	$ 349,112
Purchases	1,185,664
Principal paydowns	(297,923)	(85,526)
Net periodic amortization (accretion)	(7,846)	(1,663)
Change to loan loss provision	396	(5,291)
Balance, end of period	$ 1,136,923	$ 256,632