Available for Sale Securities by Collateral Type (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investment Holdings [Line Items]
Available-for-sale securities	$ 1,193,824	$ 1,075,581
Senior Securities
Investment Holdings [Line Items]
Available-for-sale securities	558,976	711,789
Senior Securities | Residential Prime
Investment Holdings [Line Items]
Available-for-sale securities	363,822	466,523
Senior Securities | Residential non-prime
Investment Holdings [Line Items]
Available-for-sale securities	195,154	245,266
Re-REMIC
Investment Holdings [Line Items]
Available-for-sale securities	156,346	163,035
Subordinate Securities
Investment Holdings [Line Items]
Available-for-sale securities	478,502	200,757
Subordinate Securities | Residential Prime
Investment Holdings [Line Items]
Available-for-sale securities	478,369	184,528
Subordinate Securities | Residential non-prime
Investment Holdings [Line Items]
Available-for-sale securities	133	2,160
Subordinate Securities | Commercial
Investment Holdings [Line Items]
Available-for-sale securities		$ 14,069