Schedules of Investments (Unaudited) - Investments (Details) (Parentheticals) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cost of short-term investments	$ 1,135,426,788	$ 1,303,324,809
Percentage of net asset value		96.43%
Liabilities percentage of net asset value	0.84%	3.57%
Total percentage of net asset value	100.00%	100.00%
United States Treasury Bills 2.08% – 2.25% Due 01/03/19 [Member]
U.S. Treasury bills, due date	Jan. 03, 2019
United States Treasury Bills 2.08% – 2.25% Due 01/03/19 [Member] | Minimum [Member]
U.S. Treasury bills, effective rate	2.08%
United States Treasury Bills 2.08% – 2.25% Due 01/03/19 [Member] | Maximum [Member]
U.S. Treasury bills, effective rate	2.25%
United States Treasury Bills 1.17% - 1.20% Due 01/04/18 [Member]
U.S. Treasury bills, due date		Jan. 04, 2018
United States Treasury Bills 1.17% - 1.20% Due 01/04/18 [Member] | Minimum [Member]
U.S. Treasury bills, effective rate		1.17%
United States Treasury Bills 1.17% - 1.20% Due 01/04/18 [Member] | Maximum [Member]
U.S. Treasury bills, effective rate		1.20%
United States Treasury Bills 2.32% Due 01/08/19 [Member]
U.S. Treasury bills, effective rate	2.32%
U.S. Treasury bills, due date	Jan. 08, 2019
United States Treasury Bills 1.11% - 1.21% Due 01/18/18 [Member]
U.S. Treasury bills, due date		Jan. 18, 2018
United States Treasury Bills 1.11% - 1.21% Due 01/18/18 [Member] | Minimum [Member]
U.S. Treasury bills, effective rate		1.11%
United States Treasury Bills 1.11% - 1.21% Due 01/18/18 [Member] | Maximum [Member]
U.S. Treasury bills, effective rate		1.21%
United States Treasury Bills 2.30% Due 01/10/19 [Member]
U.S. Treasury bills, effective rate	2.30%
U.S. Treasury bills, due date	Jan. 10, 2019
United States Treasury Bills 1.20% Due 01/25/18 [Member]
U.S. Treasury bills, effective rate		1.20%
U.S. Treasury bills, due date		Jan. 25, 2018
United States Treasury Bills 2.29% Due 01/15/19 [Member]
U.S. Treasury bills, effective rate	2.29%
U.S. Treasury bills, due date	Jan. 15, 2019
United States Treasury Bills 1.15% Due 02/01/18 [Member]
U.S. Treasury bills, effective rate		1.15%
U.S. Treasury bills, due date		Feb. 01, 2018
United States Treasury Bills 2.30% – 2.33% Due 01/22/19 [Member]
U.S. Treasury bills, due date	Jan. 22, 2019
United States Treasury Bills 2.30% – 2.33% Due 01/22/19 [Member] | Minimum [Member]
U.S. Treasury bills, effective rate	2.30%
United States Treasury Bills 2.30% – 2.33% Due 01/22/19 [Member] | Maximum [Member]
U.S. Treasury bills, effective rate	2.33%
United States Treasury Bills 1.31% Due 03/08/18 [Member]
U.S. Treasury bills, effective rate		1.31%
U.S. Treasury bills, due date		Mar. 08, 2018
United States Treasury Bills 2.28% Due 01/24/19 [Member]
U.S. Treasury bills, effective rate	2.28%
U.S. Treasury bills, due date	Jan. 24, 2019
United States Treasury Bills 2.40% Due 01/29/19 [Member]
U.S. Treasury bills, effective rate	2.40%
U.S. Treasury bills, due date	Jan. 29, 2019
United States Treasury Bills 2.41% Due 02/14/19 [Member]
U.S. Treasury bills, effective rate	2.41%
U.S. Treasury bills, due date	Feb. 14, 2019
US Treasury Bill Securities [Member]
Cost of short-term investments	$ 1,199,665,444	$ 1,343,671,164
Percentage of net asset value	99.16%