Statements of Changes in Net Assets - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Assets, Beginning of Year	$ 1,393,408,765	$ 1,035,799,565	$ 665,939,247
Operations:
Net investment income (loss)	12,572,317	(160,492)	(5,101,798)
Net realized gain (loss)	(11,936,763)	3,413,696	76,413,800
Net change in unrealized appreciation/depreciation	(190,844,004)	77,165,412	12,349,843
Net increase (decrease) in net assets resulting from operations	(190,208,450)	80,418,616	83,661,845
Capital Share Transactions:
Contributions for Shares issued	277,791,824	349,609,325	401,401,707
Distributions for Shares redeemed	(271,125,530)	(72,418,741)	(115,203,234)
Net increase in net assets from capital share transactions	6,666,294	277,190,584	286,198,473
Increase (decrease) in net assets	(183,542,156)	357,609,200	369,860,318
Net Assets, End of Year	$ 1,209,866,609	$ 1,393,408,765	$ 1,035,799,565
Shares issued and redeemed
Shares issued (in shares)	17,100,000	24,050,000	27,700,000
Shares redeemed (in shares)	(16,000,000)	(4,950,000)	(8,250,000)
Net increase in Shares issued and outstanding (in shares)	1,100,000	19,100,000	19,450,000