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Financial Highlights - Summary of Financial Highlights (Detail) - $ / shares
 3 Months Ended 6 Months Ended
Mar. 31, 2018
Mar. 31, 2017
Mar. 31, 2018
Mar. 31, 2017
Net Asset Value        
Net asset value per Share, beginning of period $ 123.05 $ 110.45 $ 121.91 $ 126.15
Net investment income/(loss) (0.12) (0.11) (0.25) (0.23)
Net Realized and Change in Unrealized Gain (Loss) 2.60 8.17 3.87 (7.41)
Net Income/(Loss) 2.48 8.06 3.62 (7.64)
Net asset value per Share, end of period 125.53 118.51 125.53 118.51
Market value per Share, beginning of period 123.65 109.61 121.58 125.64
Market value per Share, end of period $ 125.79 $ 118.72 $ 125.79 $ 118.72
Ratio to average net assets        
Net Investment Loss (0.40%) (0.40%) (0.40%) (0.40%)
Gross Expenses 0.40% 0.40% 0.40% 0.40%
Net Expenses 0.40% 0.40% 0.40% 0.40%
Total Return, at net asset value 2.02% 7.30% 2.97% (6.06%)
Total Return, at market value 1.73% 8.31% 3.46% (5.51%)