Trading Securities by Collateral Type (Detail) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2013	Dec. 31, 2012
Investment Holdings [Line Items]
Trading securities	$ 141,615	$ 33,172
Senior Securities
Investment Holdings [Line Items]
Trading securities	141,116	32,543
Senior Securities | Residential Prime
Investment Holdings [Line Items]
Trading securities	123,591	10,409
Senior Securities | Residential non-prime
Investment Holdings [Line Items]
Trading securities	17,525	22,134
Subordinate Securities
Investment Holdings [Line Items]
Trading securities	499	629
Subordinate Securities | Residential Prime
Investment Holdings [Line Items]
Trading securities	354	468
Subordinate Securities | Residential non-prime
Investment Holdings [Line Items]
Trading securities	$ 145	$ 161