Organization - Additional Information (Detail)	6 Months Ended
	Jun. 30, 2018 Basket shares	Dec. 31, 2017 shares	May 23, 2005 shares
Organization Consolidation And Presentation Of Financial Statements [Abstract]
General shares, shares issued	40	40	40
Treasury obligation measurement maximum maturity period	12 months
Minimum number of Baskets that can be purchased | Basket	1
Number of shares issued per basket	200,000