Summary of Realized and Unrealized Gains/(Losses) of Derivative Instruments (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended		109 Months Ended	3 Months Ended	6 Months Ended	109 Months Ended	3 Months Ended	6 Months Ended	109 Months Ended	3 Months Ended	6 Months Ended	109 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013 Realized Gain / (Losses) Exchange-traded futures contracts	Jun. 30, 2013 Realized Gain / (Losses) Exchange-traded futures contracts	Jun. 30, 2014 Realized Gain / (Losses) Exchange-traded futures contracts	Jun. 30, 2013 Unrealized Gain / (Loss) Exchange-traded futures contracts	Jun. 30, 2013 Unrealized Gain / (Loss) Exchange-traded futures contracts	Jun. 30, 2014 Unrealized Gain / (Loss) Exchange-traded futures contracts	Jun. 30, 2013 Unrealized Gain / (Loss) Forward purchase contracts	Jun. 30, 2013 Unrealized Gain / (Loss) Forward purchase contracts	Jun. 30, 2014 Unrealized Gain / (Loss) Forward purchase contracts
Schedule Of Trading Securities And Other Trading Assets [Line Items]
Derivative instrument, gain (loss)	$ (42)	$ (207)	$ 563	$ 73	$ 322	$ (235)	$ 58	$ (218)	$ 648	$ (8)	$ 12	$ (43)