Statements of Changes in Net Assets (Unaudited) - USD ($)	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017
Net Assets, Beginning of Period	$ 5,405,193,851	$ 5,541,130,643
Operations:
Net investment loss	(6,543,868)	(28,299,587)
Net realized loss	(14,286,265)	(92,120,817)
Net change in unrealized appreciation/depreciation	(168,596,426)	337,891,452
Net increase (decrease) in net assets resulting from operations	(189,426,559)	217,471,048
Capital Share Transactions:
Contributions for Shares issued	182,901,133	873,170,446
Distributions for Shares redeemed	(207,377,855)	(1,226,578,286)
Net decrease in net assets from capital share transactions	(24,476,722)	(353,407,840)
Decrease in net assets	(213,903,281)	(135,936,792)
Net Assets, End of Period	$ 5,191,290,570	$ 5,405,193,851
Shares issued and redeemed
Shares issued (in shares)	11,600,000	55,800,000
Shares redeemed (in shares)	(12,900,000)	(75,950,000)
Net decrease in Shares issued and outstanding (in shares)	(1,300,000)	(20,150,000)