Schedules of Investments (Unaudited) (Details) - Investments - USD ($)		Sep. 30, 2015	Dec. 31, 2014		Dec. 31, 2013	[2]
U.S. Treasury bills(a):
United States Treasury bills, Fair Value	[1]	$ 827,006,587	$ 784,772,217
Other Assets, Less Liabilities		(79,305,386)	2,347,549
Net Assets – 100.00%		747,701,201	787,119,766	[2],[3]	$ 1,171,092,949
United States Treasury Bills 0.01% Due 10/01/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]	89,000,000
United States Treasury bills, Fair Value	[1]	89,000,000
United States Treasury Bills 0.01% - 0.02% Due 10/08/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]	132,000,000
United States Treasury bills, Fair Value	[1]	132,000,396
United States Treasury Bills 0.00% - 0.01% Due 10/15/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]	49,000,000
United States Treasury bills, Fair Value	[1]	49,000,980
United States Treasury Bills 0.03% Due 10/22/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]	18,000,000
United States Treasury bills, Fair Value	[1]	18,000,180
United States Treasury Bills 0.01% - 0.04% Due 10/29/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]	100,000,000
United States Treasury bills, Fair Value	[1]	100,001,200
United States Treasury Bills 0.00% - 0.01% Due 12/24/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]	215,000,000
United States Treasury bills, Fair Value	[1]	215,007,525
United States Treasury Bills 0.02% Due 12/31/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]	89,000,000
United States Treasury bills, Fair Value	[1]	88,999,466
United States Treasury Bills 0.08% Due 01/07/16 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]	50,000,000
United States Treasury bills, Fair Value	[1]	49,999,150
United States Treasury Bills 0.12% - 0.13% Due 01/14/16 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]	75,000,000
United States Treasury bills, Fair Value	[1]	74,998,350
United States Treasury Bills 0.12% Due 02/04/16 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]	10,000,000
United States Treasury bills, Fair Value	[1]	9,999,340
US Treasury Bill Securities [Member]
U.S. Treasury bills(a):
United States Treasury bills, Fair Value	[1]	$ 827,006,587	784,772,217
United States Treasury Bills 0.01% - 0.06% Due 1/08/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]		32,300,000
United States Treasury bills, Fair Value	[1]		32,299,935
United States Treasury Bills 0.02% - 0.06% Due 1/15/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]		41,671,000
United States Treasury bills, Fair Value	[1]		41,670,625
United States Treasury Bills 0.06% Due 1/22/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]		85,655,000
United States Treasury bills, Fair Value	[1]		85,653,544
United States Treasury Bills 0.02% - 0.06% Due 1/29/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]		60,274,000
United States Treasury bills, Fair Value	[1]		60,272,855
United States Treasury Bills 0.01% - 0.06% Due 2/5/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]		44,568,000
United States Treasury bills, Fair Value	[1]		44,566,930
United States Treasury Bills 0.03% - 0.05% Due 2/12/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]		145,000,000
United States Treasury bills, Fair Value	[1]		144,997,100
United States Treasury Bills 0.01% - 0.05% Due 2/19/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]		66,336,000
United States Treasury bills, Fair Value	[1]		66,334,475
United States Treasury Bills 0.03% Due 3/5/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]		104,000,000
United States Treasury bills, Fair Value	[1]		103,996,880
United States Treasury Bills 0.03% - 0.05% Due 3/12/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]		137,000,000
United States Treasury bills, Fair Value	[1]		136,993,424
United States Treasury Bills 0.04% Due 4/16/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]		27,000,000
United States Treasury bills, Fair Value	[1]		26,997,273
United States Treasury Bills 0.07% Due 5/28/15 [Member]
U.S. Treasury bills(a):
United States Treasury bills, Face Amount	[1]		41,000,000
United States Treasury bills, Fair Value	[1]		$ 40,989,176

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

[2] Previously reported as "Statements of Changes in Shareholders' Capital." Please refer to Note 2A.

[3] Previously reported as "Statements of Financial Condition." Please refer to Note 2A.