Financial Highlights - Ratios to Average Limited Partners' Capital (Detail) - Limited Partners [Member]	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Ratios to Average Limited Partners' Capital:
Net investment loss	(4.10%)	(4.90%)	(4.30%)	(5.20%)
Operating expenses before incentive fees	5.40%	5.70%	5.50%	5.80%
Incentive fees	0.00%	0.00%	0.00%	0.00%
Operating expenses after incentive fees	5.40%	5.70%	5.50%	5.80%
Total return:
Total return before incentive fees	3.30%	(2.10%)	(3.40%)	(8.60%)
Incentive fees	0.00%	0.00%	0.00%	0.00%
Total return after incentive fees	3.30%	(2.10%)	(3.40%)	(8.60%)