Schedules of Investments - Investments (Details) - USD ($)		Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
U.S. Treasury bills, fair value		$ 750,519,365	$ 1,199,726,652
Other Assets, Less Liabilities		24,823,740	10,139,957
Net Assets		775,343,105	1,209,866,609	$ 1,393,408,765	$ 1,035,799,565
United States Treasury Bills 1.56% Due 01/07/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	50,000,000
U.S. Treasury bills, fair value	[1],[2]	49,990,521
United States Treasury Bills 2.08% – 2.25% Due 01/03/19 [Member]
U.S. Treasury bills, face amount	[1],[2]		16,000,000
U.S. Treasury bills, fair value	[1],[2]		15,999,000
United States Treasury Bills 1.54% - 1.87% Due 01/16/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	85,000,000
U.S. Treasury bills, fair value	[1],[2]	84,953,805
United States Treasury Bills 2.32% Due 01/08/19 [Member]
U.S. Treasury bills, face amount	[1],[2]		200,000,000
U.S. Treasury bills, fair value	[1],[2]		199,925,084
United States Treasury Bills 1.62% Due 01/21/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	50,600,000
U.S. Treasury bills, fair value	[1],[2]	50,560,743
United States Treasury Bills 2.30% Due 01/10/19 [Member]
U.S. Treasury bills, face amount	[1],[2]		145,000,000
U.S. Treasury bills, fair value	[1],[2]		144,927,380
United States Treasury Bills 1.87% due 01/23/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	50,000,000
U.S. Treasury bills, fair value	[1],[2]	49,957,125
United States Treasury Bills 2.29% Due 01/15/19 [Member]
U.S. Treasury bills, face amount	[1],[2]		200,000,000
U.S. Treasury bills, fair value	[1],[2]		199,835,784
United States Treasury Bills 1.55% Due 01/30/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	100,000,000
U.S. Treasury bills, fair value	[1],[2]	99,886,820
United States Treasury Bills 2.30% – 2.33% Due 01/22/19 [Member]
U.S. Treasury bills, face amount	[1],[2]		350,000,000
U.S. Treasury bills, fair value	[1],[2]		349,552,777
United States Treasury Bills 1.56% Due 02/04/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	15,000,000
U.S. Treasury bills, fair value	[1],[2]	14,978,480
United States Treasury Bills 2.28% Due 01/24/19 [Member]
U.S. Treasury bills, face amount	[1],[2]		180,000,000
U.S. Treasury bills, fair value	[1],[2]		179,748,925
United States Treasury Bills 1.88% – 1.93% due 02/06/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	70,000,000
U.S. Treasury bills, fair value	[1],[2]	69,898,597
United States Treasury Bills 2.40% Due 01/29/19 [Member]
U.S. Treasury bills, face amount	[1],[2]		50,000,000
U.S. Treasury bills, fair value	[1],[2]		49,909,343
United States Treasury Bills 1.55% Due 02/13/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	65,000,000
U.S. Treasury bills, fair value	[1],[2]	64,887,198
United States Treasury Bills 2.41% Due 02/14/19 [Member]
U.S. Treasury bills, face amount	[1],[2]		60,000,000
U.S. Treasury bills, fair value	[1],[2]		59,828,359
United States Treasury Bills 1.55% Due 02/20/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	30,000,000
U.S. Treasury bills, fair value	[1],[2]	29,938,138
US Treasury Bill Securities [Member]
U.S. Treasury bills, fair value		750,519,365	$ 1,199,726,652
United States Treasury Bills 1.81% Due 02/27/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	15,000,000
U.S. Treasury bills, fair value	[1],[2]	14,964,767
United States Treasury Bills 1.53% - 1.61% Due 03/12/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	75,000,000
U.S. Treasury bills, fair value	[1],[2]	74,780,885
United States Treasury Bills 1.55% Due 03/26/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	25,000,000
U.S. Treasury bills, fair value	[1],[2]	24,912,048
United States Treasury Bills 1.55% Due 04/16/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	35,000,000
U.S. Treasury bills, fair value	[1],[2]	34,845,071
United States Treasury Bills 1.55% - 1.61% Due 04/23/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	55,000,000
U.S. Treasury bills, fair value	[1],[2]	54,737,729
United States Treasury Bills 1.51% Due 05/07/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	15,000,000
U.S. Treasury bills, fair value	[1],[2]	14,920,003
United States Treasury Bills 1.55% due 05/14/20 [Member]
U.S. Treasury bills, face amount	[1],[2]	16,400,000
U.S. Treasury bills, fair value	[1],[2]	$ 16,307,435

[1] A portion of the above United States Treasury bills are posted as margin for the Trust's Index Futures positions as described in Note 2D.

[2] Rates shown are discount rates paid at the time of purchase.