Investments in Affiliates - Summary of Transactions in, and Earnings from, Investments in PowerShares Treasury Collateral Portfolio (Detail) - USD ($)		3 Months Ended		9 Months Ended
		Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Investments In And Advances To Affiliates [Line Items]
Beginning Value				$ 3,786,312
Change in Unrealized Appreciation (Depreciation)		$ 13,598	$ 39,095	83,116	$ 53,306
Ending Value		115,167,039		115,167,039
Affiliated Investments [Member]
Investments In And Advances To Affiliates [Line Items]
Ending Value		115,167,039		115,167,039
Affiliated Investments [Member] | Exchange-Traded Fund - PowerShares Treasury Collateral Portfolio [Member]
Investments In And Advances To Affiliates [Line Items]
Purchases at Cost		80,006,724		80,006,724
Change in Unrealized Appreciation (Depreciation)		121,726		121,726
Ending Value	[1],[2]	$ 80,128,450		$ 80,128,450

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

[2] Non-income producing security.