Summary of Realized and Unrealized Gains/(Losses) of Derivative Instruments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended	106 Months Ended
	Mar. 31, 2013	Mar. 31, 2014
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Derivative instruments, gain (loss)	$ (257)	$ 605
Realized Gain/(Loss) | Exchange-traded futures contracts
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Derivative instruments, gain (loss)	249	(235)
Unrealized Gain / (Loss) | Exchange-traded futures contracts
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Derivative instruments, gain (loss)	(276)	648
Unrealized Gain / (Loss) | Forward purchase contracts
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Derivative instruments, gain (loss)	$ 20	$ (43)