Financial Highlights - Ratios to Average Limited Partners' Capital (Detail) - Limited Partners [Member]	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Ratios to Average Limited Partners' Capital:
Net investment loss	(4.30%)	(5.20%)	(4.40%)	(5.30%)
Operating expenses before incentive fees	5.60%	5.80%	5.60%	5.80%
Incentive fees	0.00%	0.00%	0.00%	0.00%
Operating expenses after incentive fees	5.60%	5.80%	5.60%	5.80%
Total return:
Total return before incentive fees	(3.30%)	(4.20%)	(6.50%)	(6.60%)
Incentive fees	0.00%	0.00%	0.00%	0.00%
Total return after incentive fees	(3.30%)	(4.20%)	(6.50%)	(6.60%)