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Redeemable Capital Shares - Schedule for Redeemable Capital Shares Activity (Detail) - USD ($)
3 Months Ended
Mar. 31, 2019
Mar. 31, 2018
Temporary Equity Disclosure [Abstract]    
Opening Balance, Shares 2,250,000 2,150,000
Opening Balance $ 162,583,352 $ 169,271,222
Shares issued,shares 250,000 200,000
Shares issued $ 18,501,243 $ 15,598,271
Shares redeemed, Shares (750,000) (150,000)
Shares redeemed $ (55,749,121) $ (11,698,732)
Adjustment of redeemable capital shares to redemption value $ 3,964,886 $ (4,806,853)
Ending Balance, Shares 1,750,000 2,250,000
Ending Balance $ 129,300,360 $ 168,363,908