Statements of Financial Condition (Unaudited) (Parenthetical) - USD ($)	Sep. 30, 2017	Dec. 31, 2016
Investments, cost	$ 643,396,721	$ 849,976,394
Affiliated Investments [Member]
Affiliated Investments, at Cost	115,045,313	3,786,312
US Treasury Bill Securities [Member]
Investments, cost	$ 528,351,408	$ 846,190,082