Statements of Financial Condition (Parenthetical) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Investments, cost	$ 2,157,140,963	$ 2,534,934,371
US Treasury Bill Securities [Member]
Investments, cost	1,839,147,058	2,403,904,566
Affiliated Investments [Member]
Affiliated Investments, at Cost	$ 317,993,905	$ 131,029,805