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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 1,650,000 1,700,000
Shares Issued, Shares 100,000  
Shares Redeemed, Shares (250,000) (50,000)
Ending Balance, Shares 1,500,000 1,650,000
Opening Balance, Amount $ 116,248,970 $ 133,025,963
Shares Issued, Amount 7,094,912  
Shares Redeemed, Amount (17,792,563) (3,929,418)
Adjustment of Redeemable Capital Shares to Redemption Value 1,075,454 (2,440,921)
Ending Balance, Amount $ 106,626,773 $ 126,655,624