Trust Expenses (Narrative) (Details) (USD $)	6 Months Ended
	Jun. 30, 2011
Trust Expenses
Sponsor's fee, annualized accrual rate based on adjusted daily net asset value prior to adjustment date	0.40%
Sponsor's fee, annualized accrual rate based on adjusted daily net asset value, after adjustment date	0.25%
Sponsor's fee adjustment date	July 1, 2010
Legal fees, per year, maximum	$ 100,000