Statements of Financial Condition (Unaudited) (Parenthetical) - USD ($)	Mar. 31, 2019	Dec. 31, 2018
Investments in Securities, cost	$ 459,583,199	$ 499,901,020
US Treasury Bill Securities [Member]
Investments in Securities, cost	361,795,667	405,208,278
Affiliated Investments [Member]
Affiliated Investments, at Cost	$ 97,787,532	$ 94,692,742