Statements of Changes in Net Assets - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Assets, Beginning of Year	$ 5,405,193,851	$ 5,541,130,643	$ 4,391,943,604
Operations:
Net investment loss	(25,335,523)	(28,299,587)	(29,148,568)
Net realized loss	(135,779,475)	(92,120,817)	(111,239,707)
Net change in unrealized appreciation/depreciation	(311,211,423)	337,891,452	823,171,043
Net increase (decrease) in net assets resulting from operations	(472,326,421)	217,471,048	682,782,768
Capital Share Transactions:
Contributions for Shares issued	954,051,929	873,170,446	1,702,628,324
Distributions for Shares redeemed	(982,882,736)	(1,226,578,286)	(1,236,224,053)
Net increase (decrease) in net assets from capital share transactions	(28,830,807)	(353,407,840)	466,404,271
Increase (decrease) in net assets	(501,157,228)	(135,936,792)	1,149,187,039
Net Assets, End of Year	$ 4,904,036,623	$ 5,405,193,851	$ 5,541,130,643
Shares issued and redeemed
Shares issued (in shares)	64,000,000	55,800,000	102,750,000
Shares redeemed (in shares)	(65,900,000)	(75,950,000)	(76,400,000)
Net increase (decrease) in Shares issued and outstanding (in shares)	(1,900,000)	(20,150,000)	26,350,000