Trading Activities - Additional Information (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Schedule of Trading Securities and Other Trading Assets [Line Items]
Average number of futures and exchange-cleared swap contracts traded	45,323	27,942	46,021	41,054
Average number of option contracts traded	58,260	71,373	57,321	77,841
Futures [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative instrument, service fee per transaction			$ 0.70
Swaps [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative instrument, service fee per transaction			0.70
Option [Member]
Schedule of Trading Securities and Other Trading Assets [Line Items]
Derivative instrument, service fee per transaction			$ 0.35