Statements of Financial Condition (Unaudited) (Parenthetical) - USD ($)	Mar. 31, 2018	Dec. 31, 2017
Investments in Securities, cost	$ 500,717,746	$ 541,547,464
Affiliated Investments [Member]
Affiliated Investments, at Cost	97,593,316	81,329,032
US Treasury Bill Securities [Member]
Investments in Securities, cost	$ 403,124,430	$ 460,218,432