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Statements of Changes in Net Assets (Unaudited) - USD ($)
 9 Months Ended 12 Months Ended
Sep. 30, 2018
Dec. 31, 2017
Net Assets, Beginning of Period $ 5,405,193,851 $ 5,541,130,643
Operations:    
Net investment loss (19,371,652) (28,299,587)
Net realized loss (68,160,375) (92,120,817)
Net change in unrealized appreciation/depreciation (750,921,945) 337,891,452
Net increase (decrease) in net assets resulting from operations (838,453,972) 217,471,048
Capital Share Transactions:    
Contributions for Shares issued 880,252,487 873,170,446
Distributions for Shares redeemed (684,717,667) (1,226,578,286)
Net increase (decrease) in net assets from capital share transactions 195,534,820 (353,407,840)
Decrease in net assets (642,919,152) (135,936,792)
Net Assets, End of Period $ 4,762,274,699 $ 5,405,193,851
Shares issued and redeemed    
Shares issued (in shares) 58,600,000 55,800,000
Shares redeemed (in shares) (44,100,000) (75,950,000)
Net increase (decrease) in Shares issued and outstanding (in shares) 14,500,000 (20,150,000)