Trading Securities by Collateral Type (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2013	Dec. 31, 2012
Investment Holdings [Line Items]
Trading securities	$ 130,854	$ 33,172
Senior Securities
Investment Holdings [Line Items]
Trading securities	126,307	32,543
Senior Securities | Residential Prime
Investment Holdings [Line Items]
Trading securities	117,443	10,409
Senior Securities | Residential non-prime
Investment Holdings [Line Items]
Trading securities	8,864	22,134
Subordinate Securities
Investment Holdings [Line Items]
Trading securities	4,547	629
Subordinate Securities | Residential Prime
Investment Holdings [Line Items]
Trading securities	4,547	468
Subordinate Securities | Residential non-prime
Investment Holdings [Line Items]
Trading securities		$ 161