Schedules of Investments (Unaudited) - Investments (Details) - USD ($)		Dec. 31, 2018	Dec. 31, 2017		Dec. 31, 2016	Dec. 31, 2015
U.S. Treasury bills, fair value			$ 1,343,654,642
Other Assets, Less Liabilities		$ 10,139,957	49,754,123
Net Assets		1,209,866,609	1,393,408,765		$ 1,035,799,565	$ 665,939,247
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 1.17% - 1.20% Due 01/04/18 [Member]
U.S. Treasury bills, face amount	[1],[2]		116,000,000
U.S. Treasury bills, fair value	[1],[2]		115,992,693
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 1.11% - 1.21% Due 01/18/18 [Member]
U.S. Treasury bills, face amount	[1],[2]		144,000,000
U.S. Treasury bills, fair value	[1],[2]		143,920,719
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 1.20% Due 01/25/18 [Member]
U.S. Treasury bills, face amount	[1],[2]		142,000,000
U.S. Treasury bills, fair value	[1],[2]		141,888,412
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 1.15% Due 02/01/18 [Member]
U.S. Treasury bills, face amount	[1],[2]		434,500,000
U.S. Treasury bills, fair value	[1],[2]		434,041,811
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 1.31% Due 03/08/18 [Member]
U.S. Treasury bills, face amount	[1],[2]		509,000,000
U.S. Treasury bills, fair value	[1],[2]		507,811,007
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 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 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
U.S. Treasury bills, fair value		1,199,726,652
US Treasury Bill Securities [Member]
U.S. Treasury bills, fair value		$ 1,199,726,652	$ 1,343,654,642	[1],[2]

[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.