Statements of Financial Condition (Parenthetical) - USD ($)	Dec. 31, 2017	Dec. 31, 2016
Investments, cost	$ 639,590,621	$ 721,689,108
Affiliated Investments [Member]
Affiliated Investments, at Cost	124,826,575	19,456,137
US Treasury Bill Securities [Member]
Investments, cost	$ 514,764,046	$ 702,232,971