Statements of Changes in Net Assets (Unaudited) - USD ($)	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Net Assets, Beginning of Period	$ 4,668,431,637	$ 4,904,036,623	$ 5,191,290,570	$ 5,405,193,851	$ 4,904,036,623	$ 5,405,193,851
Operations:
Net investment loss	(5,821,811)	(5,951,914)	(6,592,413)	(6,543,868)	(11,773,725)	(13,136,281)
Net realized loss	(17,462,196)	(43,054,318)	(28,206,317)	(14,286,265)	(60,516,514)	(42,492,582)
Net change in unrealized appreciation/depreciation	57,866,685	(69,197,383)	(43,957,463)	(168,596,426)	(11,330,698)	(212,553,889)
Net increase (decrease) in net assets resulting from operations	34,582,678	(118,203,615)	(78,756,193)	(189,426,559)	(83,620,937)	(268,182,752)
Capital Share Transactions:
Contributions for Shares issued	300,557,309	185,279,712	404,809,725	182,901,133
Distributions for Shares redeemed	(84,431,118)	(302,681,083)	(350,390,495)	(207,377,855)
Net decrease in net assets from capital share transactions	216,126,191	(117,401,371)	54,419,230	(24,476,722)
Increase (decrease) in net assets	250,708,869	(235,604,986)	(24,336,963)	(213,903,281)
Net Assets, End of Period	$ 4,919,140,506	$ 4,668,431,637	$ 5,166,953,607	$ 5,191,290,570	$ 4,919,140,506	$ 5,166,953,607
Shares issued and redeemed
Shares issued (in shares)					34,150,000	37,900,000
Shares redeemed (in shares)					(26,800,000)	(35,100,000)
Net increase in Shares issued and outstanding (in shares)					7,350,000	2,800,000