Statements of Financial Condition (Unaudited) (Parenthetical) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Investments in Securities, Cost	$ 372,926,198	$ 511,790,491
US Treasury Bill Securities [Member]
Investments in Securities, Cost	273,516,261	409,385,046
Affiliated Investments [Member]
Affiliated Investments, at Cost	$ 99,409,937	$ 102,405,445