Financial Performance (Detail) (USD $)	3 Months Ended		6 Months Ended
	Jun. 30, 2012	Jun. 30, 2011	Jun. 30, 2012	Jun. 30, 2011
Class A
Per share operating performance:
Total return before incentive fees	(2.81%)	(4.75%)	(8.09%)	(8.19%)
Less incentive fee allocations	(0.02%)	(0.14%)	(0.01%)	(0.26%)
Total return	(2.83%)	(4.89%)	(8.10%)	(8.45%)
Ratios to average net assets:
Trading income (loss)	(0.75%)	(4.94%)	(4.51%)	(8.74%)
Expenses:
Expenses, less incentive fees	(2.13%)	(2.40%)	(4.34%)	(4.56%)
Less incentive fee allocations	(0.02%)	(0.14%)	(0.01%)	(0.26%)
Total expenses	(2.15%)	(2.54%)	(4.35%)	(4.82%)
Class A | Trading
Per share operating performance:
Net asset value of Trading units, beginning of period	86.37	96.88	91.32	100.64
Trading gain (loss)	(0.71)	(2.42)	(3.86)	(4.07)
Investment income	0.08	0.11	0.22	0.24
Expenses	(1.81)	(2.43)	(3.75)	(4.67)
Trading income (loss)	(2.44)	(4.74)	(7.39)	(8.50)
Net asset value of Trading units, end of period	83.93	92.14	83.93	92.14
Class B
Per share operating performance:
Total return before incentive fees	(2.32%)	(4.28%)	(7.14%)	(7.29%)
Less incentive fee allocations	(0.02%)	(0.13%)	(0.02%)	(0.24%)
Total return	(2.34%)	(4.41%)	(7.16%)	(7.53%)
Ratios to average net assets:
Trading income (loss)	(0.64%)	(4.68%)	(4.48%)	(7.99%)
Expenses:
Expenses, less incentive fees	(1.62%)	(1.93%)	(3.33%)	(3.56%)
Less incentive fee allocations	(0.02%)	(0.13%)	(0.02%)	(0.24%)
Total expenses	(1.64%)	(2.06%)	(3.35%)	(3.80%)
Class B | Trading
Per share operating performance:
Net asset value of Trading units, beginning of period	92.18	101.34	96.98	104.75
Trading gain (loss)	(0.76)	(2.75)	(4.11)	(4.52)
Investment income	0.09	0.12	0.23	0.25
Expenses	(1.48)	(1.84)	(3.07)	(3.61)
Trading income (loss)	(2.15)	(4.47)	(6.95)	(7.88)
Net asset value of Trading units, end of period	90.03	96.87	90.03	96.87