Investments in Affiliates - Summary of Transactions in, and Earnings from, Investments in PowerShares Treasury Collateral Portfolio (Detail) - USD ($)		3 Months Ended		6 Months Ended
		Jun. 30, 2017	Jun. 30, 2016	Jun. 30, 2017	Jun. 30, 2016
Investments in and Advances to Affiliates [Line Items]
Beginning Value				$ 131,029,805
Change in Unrealized Appreciation (Depreciation)		$ 266,321	$ 195,958	150,092	$ 374,068
Ending Value		346,560,479		346,560,479
Affiliated Investments [Member]
Investments in and Advances to Affiliates [Line Items]
Ending Value		346,560,479		346,560,479
Affiliated Investments [Member] | Exchange-Traded Fund - PowerShares Treasury Collateral Portfolio [Member]
Investments in and Advances to Affiliates [Line Items]
Beginning Value		250,074,684
Purchases at Cost				249,990,226
Change in Unrealized Appreciation (Depreciation)		450,756		535,214
Ending Value	[1],[2]	$ 250,525,440		$ 250,525,440

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

[2] Non-income producing security.