Financial Highlights - Financial Highlights for Limited Partner Class (Detail) - Class A [Member] - $ / 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	$ 8.55	$ 8.84
Net asset value per Unit, end of year	8.71	8.55	$ 8.84
Limited Partners [Member]
Per Unit Performance (for a unit outstanding throughout the year):
Net realized and unrealized gains (losses)	0.50	0.12	(0.52)
Net investment loss	(0.34)	(0.41)	(0.56)
Increase (decrease) for the year	0.16	(0.29)	(1.08)
Net asset value per Unit, beginning of year	8.55	8.84	9.92
Net asset value per Unit, end of year	$ 8.71	$ 8.55	$ 8.84
Ratios to Average Limited Partners' Capital:
Net investment loss	(4.00%)	(4.50%)	(5.80%)
Operating expenses before expenses borne by the General Partner and incentive fees	7.60%	5.20%	5.20%
Expenses borne by the General Partner	(3.00%)
Incentive fees	0.90%		0.80%
Operating expenses after expenses borne by the General Partner and incentive fees	5.50%	5.20%	6.00%
Total return:
Total return before incentive fees	2.80%	(3.30%)	(10.10%)
Incentive fees	(0.90%)		(0.80%)
Total return after incentive fees	1.90%	(3.30%)	(10.90%)