Derivative Financial Instruments - Balance Sheet Information (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Financial Instruments Not Designated as Hedging Instruments, Unrealized Gain / (Loss)	$ 50	$ 22
Investments-trading [Member] | TBAs [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Financial Instruments Not Designated as Hedging Instruments, Unrealized Gain / (Loss)	120	35
Trading securities sold, not yet purchased [Member] | TBAs [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Financial Instruments Not Designated as Hedging Instruments, Unrealized Gain / (Loss)	$ (70)	$ (13)