Available for Sale Securities by Collateral Type (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investment Holdings [Line Items]
Available-for-sale securities	$ 1,172,690	$ 1,075,581
Senior Securities
Investment Holdings [Line Items]
Available-for-sale securities	634,846	711,789
Senior Securities | Residential Prime
Investment Holdings [Line Items]
Available-for-sale securities	410,159	466,523
Senior Securities | Residential non-prime
Investment Holdings [Line Items]
Available-for-sale securities	224,687	245,266
Re-REMIC
Investment Holdings [Line Items]
Available-for-sale securities	154,167	163,035
Subordinate Securities
Investment Holdings [Line Items]
Available-for-sale securities	383,677	200,757
Subordinate Securities | Residential Prime
Investment Holdings [Line Items]
Available-for-sale securities	381,631	184,528
Subordinate Securities | Residential non-prime
Investment Holdings [Line Items]
Available-for-sale securities	2,046	2,160
Subordinate Securities | Commercial
Investment Holdings [Line Items]
Available-for-sale securities		$ 14,069