Schedule of Investments (Unaudited) - Treasury Bills and Money Market Fund - USD ($)		3 Months Ended		12 Months Ended
		Mar. 31, 2018		Dec. 31, 2017
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity		99.96%		101.59%
Value		$ 403,014,990		$ 460,172,505
Affiliated Investments, at value		97,586,352		81,011,698
Total Investments in Securities		500,601,342		541,184,203
Investments in Securities, cost		$ 500,717,746		$ 541,547,464
Affiliated Investments [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity		19.49%		15.21%
Affiliated Investments, at value		$ 97,586,352		$ 81,011,698
Affiliated Investments, at Cost		$ 97,593,316		$ 81,329,032
U.S. Treasury Bills, 1.695% due May 3, 2018 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity	[1]	8.77%
Value	[1]	$ 43,938,593
Principal Value	[1]	$ 44,000,000
Investment Interest Rate	[1]	1.695%
Investment Maturity Date	[1]	May 03, 2018
U.S. Treasury Bills, 1.660% due June 7, 2018 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity	[1]	15.73%
Value	[1]	$ 78,760,045
Principal Value	[1]	$ 79,000,000
Investment Interest Rate	[1]	1.66%
Investment Maturity Date	[1]	Jun. 07, 2018
U.S. Treasury Bills, 1.740% due July 5, 2018 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity	[1],[2]	30.21%
Value	[1],[2]	$ 151,323,167
Principal Value	[1],[2]	$ 152,000,000
Investment Interest Rate	[1],[2]	1.74%
Investment Maturity Date	[1],[2]	Jul. 05, 2018
Restricted for maintenance margin		$ 19,910,000
U.S. Treasury Bills, 1.830% due August 30, 2018 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity	[1]	25.76%
Value	[1]	$ 128,993,185
Principal Value	[1]	$ 130,000,000
Investment Interest Rate	[1]	1.83%
Investment Maturity Date	[1]	Aug. 30, 2018
United States Treasury Obligations [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity	[1]	80.47%		86.38%
Value	[1]	$ 403,014,990		$ 460,172,505
Investments in Securities, cost	[1]	$ 403,124,430		$ 460,218,432
Exchange-Traded Fund - PowerShares Treasury Collateral Portfolio [Member] | Affiliated Investments [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity	[3]	15.98%		14.96%
Affiliated Investments, at value	[3]	$ 79,999,760		$ 79,689,390
Shares	[3]	757,000		757,000
Affiliated Investments, at Cost	[3]	$ 80,006,724		$ 80,006,724
Money Market Mutual Fund - Invesco Premier U.S. Government Money Portfolio - Institutional Class [Member] | Affiliated Investments [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity		3.51%	[4]	0.25%	[5]
Affiliated Investments, at value		$ 17,586,592	[4]	$ 1,322,308	[5]
Shares		17,586,592	[4]	1,322,308	[5]
Investment Interest Rate		1.52%	[4]	1.15%	[5]
Investments in Securities, cost		$ 17,586,592	[4]	$ 1,322,308	[5]
U.S. Treasury Bills, 1.180% due January 4, 2018 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity	[1]			35.10%
Value	[1]			$ 186,988,273
Principal Value	[1]			$ 187,000,000
Investment Interest Rate	[1]			1.18%
Investment Maturity Date	[1]			Jan. 04, 2018
U.S. Treasury Bills, 1.245% due January 18, 2018 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity	[1]			4.50%
Value	[1]			$ 23,986,840
Principal Value	[1]			$ 24,000,000
Investment Interest Rate	[1]			1.245%
Investment Maturity Date	[1]			Jan. 18, 2018
U.S. Treasury Bills, 1.285% due March 1, 2018 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity	[1],[6]			34.10%
Value	[1],[6]			$ 181,628,342
Principal Value	[1],[6]			$ 182,000,000
Investment Interest Rate	[1],[6]			1.285%
Investment Maturity Date	[1],[6]			Mar. 01, 2018
Restricted for maintenance margin				$ 19,958,000
U.S. Treasury Bills, 1.450% due June 7, 2018 [Member]
Schedule of Investments [Line Items]
Percentage of Shareholders' Equity	[1]			12.68%
Value	[1]			$ 67,569,050
Principal Value	[1]			$ 68,000,000
Investment Interest Rate	[1]			1.45%
Investment Maturity Date	[1]			Jun. 07, 2018

[1] Security may be traded on a discount basis. The interest rate shown represents the discount rate at the most recent auction date of the security prior to period end.

[2] United States Treasury Obligations of $19,910,000 are on deposit with the Commodity Broker and held as maintenance margin for open futures contracts.

[3] Affiliated company. The security and the Fund are affiliated by having the same investment adviser. See Note 8.

[4] The security and the Fund are advised by wholly-owned subsidiaries of Invesco Ltd. and are therefore considered to be affiliated. The rate shown is the 7-day SEC standardized yield as of March 31, 2018.

[5] The security and the Fund are advised by wholly-owned subsidiaries of Invesco Ltd. and are therefore considered to be affiliated. The rate shown is the 7-day SEC standardized yield as of December 31, 2017.

[6] United States Treasury Obligations of $19,958,000 are on deposit with the Commodity Broker and held as maintenance margin for open futures contracts.