Significant Accounting Policies - Additional Information (Detail) - USD ($) |
9 Months Ended | 12 Months Ended | |
|---|---|---|---|
Jul. 31, 2014 |
Sep. 30, 2017 |
Dec. 31, 2016 |
|
| Significant Accounting Policies [Line Items] | |||
| Level III portfolio investments | $ 0 | $ 0 | |
| Minimum [Member] | Volatility Index Fund [Member] | |||
| Significant Accounting Policies [Line Items] | |||
| Percentage of brokerage commissions exceed variable create/redeem fees | 0.02% |