Financial Highlights - Ratios to Average Limited Partners' Capital (Detail) - Limited Partners [Member]	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Ratios to Average Limited Partners' Capital:
Net investment loss	(4.50%)	(5.40%)
Operating expenses before incentive fees	5.60%	5.90%
Incentive fees	0.00%	0.00%
Operating expenses after incentive fees	5.60%	5.90%
Total return:
Total return before incentive fees	(3.40%)	(2.50%)
Incentive fees	0.00%	0.00%
Total return after incentive fees	(3.40%)	(2.50%)