Organization - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2018 Basket shares	Dec. 31, 2017 shares	Feb. 23, 2015 shares	Aug. 03, 2006 Trust
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Number of series of trust | Trust				7
General shares, shares issued	40	40	40
Purchase agreement transaction date	Feb. 23, 2015
Treasury obligation measurement maximum maturity period	12 months
Number of shares issued per basket	200,000
Minimum number of Baskets that can be purchased | Basket	1