Financial Highlights - Financial Highlights for the Limited Partner Class (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Per Unit Performance (for a unit outstanding throughout the year):
Net asset value per Unit, beginning of year	$ 23.82
Net asset value per Unit, end of year	20.45	$ 23.82
Limited Partners [Member]
Per Unit Performance (for a unit outstanding throughout the year):
Net realized and unrealized gains (losses)	(2.42)	1.42	$ (2.96)
Net investment loss	(0.95)	(1.14)	(1.43)
Net increase (decrease) for the year	(3.37)	0.28	(4.39)
Net asset value per Unit, beginning of year	23.82	23.54	27.93
Net asset value per Unit, end of year	$ 20.45	$ 23.82	$ 23.54
Ratios to Average Limited Partners' Capital:
Net investment loss	(4.20%)	(5.00%)	(5.50%)
Operating expenses before incentive fees	5.50%	5.70%	5.70%
Incentive fees	0.00%	0.00%	0.00%
Operating expenses after incentive fees	5.50%	5.70%	5.70%
Total return:
Total return before incentive fees	(14.10%)	1.20%	(15.70%)
Incentive fees	(0.00%)	(0.00%)	(0.00%)
Total return after incentive fees	(14.10%)	1.20%	(15.70%)