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Financial Highlights (Parenthetical)
3 Months Ended 6 Months Ended
Jun. 30, 2017
Jun. 30, 2016
Jun. 30, 2017
Jun. 30, 2016
Financial Highlights [Abstract]        
Gross Annualized Sponsor's Fee per share 0.45% 0.45% 0.45% 0.45%