Financial Highlights - Summary of Ratios to Average Net Assets (Detail)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Class A [Member]
Ratios to Average Net Assets:
Ratios to Average Net Assets, Net investment income (loss)	(6.40%)	(6.10%)	(6.10%)
Ratios to Average Net Assets, Net investment income (loss), Allocation to Special Limited Partner	(0.70%)	0.00%	0.00%
Ratios to Average Net Assets, Net investment income (loss) after allocation to Special Limited Partner	(7.10%)	(6.10%)	(6.10%)
Ratios to Average Net Assets, Operating expenses	6.80%	6.30%	6.10%
Ratios to Average Net Assets, Operating expenses, Allocation to Special Limited Partner	0.70%	0.00%	0.00%
Ratios to Average Net Assets, Total expenses	7.50%	6.30%	6.10%
Total return :
Total return before allocation to Special Limited Partner	2.30%	4.40%	2.10%
Total return, Allocation to Special Limited Partner	(0.80%)	0.00%	0.00%
Total return after allocation to Special Limited Partner	1.50%	4.40%	2.10%
Class D [Member]
Ratios to Average Net Assets:
Ratios to Average Net Assets, Net investment income (loss)	(6.60%)	(4.70%)	(5.40%)
Ratios to Average Net Assets, Net investment income (loss), Allocation to Special Limited Partner	(1.60%)	(0.30%)	0.00%
Ratios to Average Net Assets, Net investment income (loss) after allocation to Special Limited Partner	(8.20%)	(5.00%)	(5.40%)
Ratios to Average Net Assets, Operating expenses	7.20%	4.90%	5.40%
Ratios to Average Net Assets, Operating expenses, Allocation to Special Limited Partner	1.60%	0.30%	0.00%
Ratios to Average Net Assets, Total expenses	8.80%	5.20%	5.40%
Total return :
Total return before allocation to Special Limited Partner	4.10%	5.60%	3.40%
Total return, Allocation to Special Limited Partner	(1.60%)	(0.20%)	0.00%
Total return after allocation to Special Limited Partner	2.50%	5.40%	3.40%