Summary of Realized and Unrealized Gains/(Losses) of Derivative Instruments (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Derivative Instruments Gain Loss [Line Items]
Derivative instrument, gain (loss)				$ (175)
Exchange-traded futures contracts
Derivative Instruments Gain Loss [Line Items]
Derivative instrument, gain (loss)	0	42	0	(176)
Forward purchase contracts
Derivative Instruments Gain Loss [Line Items]
Derivative instrument, gain (loss)	0	(11)	0	1
Realized Gain / (Losses) | Exchange-traded futures contracts
Derivative Instruments Gain Loss [Line Items]
Derivative instrument, gain (loss)	$ 0	$ 168	$ 0	$ 490