Statements of Financial Condition (Unaudited) (Parenthetical) - USD ($)	Jun. 30, 2018	Dec. 31, 2017
Investments in Securities, cost	$ 327,103,049	$ 283,534,397
US Treasury Bill Securities [Member]
Investments in Securities, cost	295,916,080	244,424,302
Affiliated Investments [Member]
Affiliated Investments, at Cost	$ 31,186,969	$ 39,110,095