Statements of Cash Flows - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Cash Flows from Operating Activities
Net increase (decrease) in net assets resulting from operations	$ (190,208,450)	$ 80,418,616	$ 83,661,845
Adjustments to reconcile net increase (decrease) in net assets resulting from operations to net cash provided by (used in) operating activities:
Purchases of short-term investments	(14,886,123,153)	(7,773,697,622)	(5,383,498,689)
Sales/maturities of short-term investments	15,054,622,653	7,442,051,201	5,035,429,323
Accretion of discount	(24,490,437)	(9,702,292)	(2,294,182)
Net realized (gain) loss on short-term investments	(3,343)	(2,812)	(50,156)
Net change in unrealized appreciation/depreciation on short-term investments	(77,730)	59,334	(20,363)
Change in operating assets and liabilities:
Receivable for variation margin on open futures contracts	4,637,434	(5,558,077)	6,701,406
Payable for variation margin on open futures contracts		(1,187,213)	1,187,213
Sponsor’s fees payable	(41,821)	206,124	211,342
Brokerage commissions and fees payable	2,171	(5,102)	10,977
Net cash provided by (used in) operating activities	(41,682,676)	(267,417,843)	(258,661,284)
Cash Flows from Financing Activities
Contributions for Shares issued	281,055,224	346,345,925	401,401,707
Distributions for Shares redeemed	(271,125,530)	(72,418,741)	(115,203,234)
Net cash provided by (used in) financing activities	9,929,694	273,927,184	286,198,473
Net increase (decrease) in cash and cash equivalents	(31,752,982)	6,509,341	27,537,189
Cash
Beginning of period, Unrestricted — cash	41,786,279	35,276,938	7,739,749
End of period, Unrestricted — cash	$ 10,033,297	$ 41,786,279	$ 35,276,938