Trust Expenses (Detail) (USD $)	12 Months Ended
	Dec. 31, 2012
Sponsor's fee, annualized accrual rate based on daily net asset value, after adjustment date	0.25%
Sponsor's fee, annualized accrual rate based on daily net asset value, prior to adjustment date	0.40%
Sponsor's fee adjustment date	July 1, 2010
Legal fees, per year, maximum	$ 100,000