Statements of Financial Condition (Parenthetical) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Investments in Securities, Cost	$ 511,790,491	$ 541,547,464
US Treasury Bill Securities [Member]
Investments in Securities, Cost	409,385,046	460,218,432
Affiliated Investments [Member]
Affiliated Investments, at Cost	$ 102,405,445	$ 81,329,032