Trading Activities - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2018 Contract	Dec. 31, 2017 USD ($)
Derivative [Line Items]
Direct investments | $		$ 0
Futures Contracts [Member]
Derivative [Line Items]
Average number of derivative contracts traded	44
Option Contracts [Member]
Derivative [Line Items]
Average number of derivative contracts traded	2