Statements of Financial Condition (Parenthetical) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Investments in Securities, cost	$ 2,009,415,674	$ 2,157,140,963
US Treasury Bill Securities [Member]
Investments in Securities, cost	1,614,486,381	1,839,147,058
Affiliated Investments [Member]
Affiliated Investments, at Cost	$ 394,929,293	$ 317,993,905