Statements of Financial Condition (Unaudited) (Parenthetical) - USD ($)	Jun. 30, 2015	Dec. 31, 2014
Restricted investments	$ 43,823,219	$ 18,366,219
Total United States Treasury Obligations [Member]
United States Treasury Obligations, cost	$ 216,996,933	$ 236,993,093

Investments which are not defined as or included in marketable (debt, equity, or other) securities whose use is restricted in whole or in part, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

No definition available.

Name:	us-gaap_RestrictedInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant