Schedules of Investments - Investments (Details) (Parentheticals) - USD ($)	Dec. 31, 2019	Dec. 31, 2018
Cost of short-term investments	$ 720,991,393	$ 1,135,426,788
Liabilities percentage of net asset value	3.20%	0.84%
Total percentage of net asset value	100.00%	100.00%
United States Treasury Bills 1.56% Due 01/07/20 [Member]
U.S. Treasury bills, effective rate	1.56%
U.S. Treasury bills, due date	Jan. 07, 2020
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 1.54% - 1.87% Due 01/16/20 [Member]
U.S. Treasury bills, due date	Jan. 16, 2020
United States Treasury Bills 1.54% - 1.87% Due 01/16/20 [Member] | Minimum [Member]
U.S. Treasury bills, effective rate	1.54%
United States Treasury Bills 1.54% - 1.87% Due 01/16/20 [Member] | Maximum [Member]
U.S. Treasury bills, effective rate	1.87%
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 1.62% Due 01/21/20 [Member]
U.S. Treasury bills, effective rate	1.62%
U.S. Treasury bills, due date	Jan. 21, 2020
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 1.87% due 01/23/20 [Member]
U.S. Treasury bills, effective rate	1.87%
U.S. Treasury bills, due date	Jan. 23, 2020
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 1.55% Due 01/30/20 [Member]
U.S. Treasury bills, effective rate	1.55%
U.S. Treasury bills, due date	Jan. 30, 2020
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 1.56% Due 02/04/20 [Member]
U.S. Treasury bills, effective rate	1.56%
U.S. Treasury bills, due date	Feb. 04, 2020
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 1.88% – 1.93% due 02/06/20 [Member]
U.S. Treasury bills, due date	Feb. 06, 2020
United States Treasury Bills 1.88% – 1.93% due 02/06/20 [Member] | Minimum [Member]
U.S. Treasury bills, effective rate	1.88%
United States Treasury Bills 1.88% – 1.93% due 02/06/20 [Member] | Maximum [Member]
U.S. Treasury bills, effective rate	1.93%
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 1.55% Due 02/13/20 [Member]
U.S. Treasury bills, effective rate	1.55%
U.S. Treasury bills, due date	Feb. 13, 2020
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
United States Treasury Bills 1.55% Due 02/20/20 [Member]
U.S. Treasury bills, effective rate	1.55%
U.S. Treasury bills, due date	Feb. 20, 2020
US Treasury Bill Securities [Member]
Cost of short-term investments	$ 750,419,242	$ 1,199,665,444
Percentage of net asset value	96.80%	96.80%
United States Treasury Bills 1.81% Due 02/27/20 [Member]
U.S. Treasury bills, effective rate	1.81%
U.S. Treasury bills, due date	Feb. 27, 2020
United States Treasury Bills 1.53% - 1.61% Due 03/12/20 [Member]
U.S. Treasury bills, due date	Mar. 12, 2020
United States Treasury Bills 1.53% - 1.61% Due 03/12/20 [Member] | Minimum [Member]
U.S. Treasury bills, effective rate	1.53%
United States Treasury Bills 1.53% - 1.61% Due 03/12/20 [Member] | Maximum [Member]
U.S. Treasury bills, effective rate	1.61%
United States Treasury Bills 1.55% Due 03/26/20 [Member]
U.S. Treasury bills, effective rate	1.55%
U.S. Treasury bills, due date	Mar. 26, 2020
United States Treasury Bills 1.55% Due 04/16/20 [Member]
U.S. Treasury bills, effective rate	1.55%
U.S. Treasury bills, due date	Apr. 16, 2020
United States Treasury Bills 1.55% - 1.61% Due 04/23/20 [Member]
U.S. Treasury bills, due date	Apr. 23, 2020
United States Treasury Bills 1.55% - 1.61% Due 04/23/20 [Member] | Minimum [Member]
U.S. Treasury bills, effective rate	1.55%
United States Treasury Bills 1.55% - 1.61% Due 04/23/20 [Member] | Maximum [Member]
U.S. Treasury bills, effective rate	1.61%
United States Treasury Bills 1.51% Due 05/07/20 [Member]
U.S. Treasury bills, effective rate	1.51%
U.S. Treasury bills, due date	May 07, 2020
United States Treasury Bills 1.55% due 05/14/20 [Member]
U.S. Treasury bills, effective rate	1.55%
U.S. Treasury bills, due date	May 14, 2020