Fair Value Measurements - Derivative Instrument Fair Value Based on Hierarchy (Detail)	Dec. 31, 2018 USD ($)
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	$ 15,181
Total liabilities	25,826
Futures Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	15,181
Total liabilities	25,826
Level 1 [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	15,181
Total liabilities	25,826
Level 1 [Member] | Futures Contracts [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Total assets	15,181
Total liabilities	$ 25,826