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Organization and Business (Additional Information) (Details) (USD $)	0 Months Ended
	Dec. 06, 2007	Dec. 04, 2007	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Fee paid by Authorized Purchasers for each order placed to create one or more Creation Baskets or to redeem one or more baskets			350usl_FeePaidPerOrder
Number of initially registered units on Form S-1 with the U.S. Securities and Exchange Commission		11,000,000usl_NumberOfUnitsRegisteredInPeriod
Net asset value per share	$ 50.00usl_NetAssetValuePerUnit		$ 23.96usl_NetAssetValuePerUnit	$ 26.59usl_NetAssetValuePerUnit	$ 44.10usl_NetAssetValuePerUnit	$ 42.83usl_NetAssetValuePerUnit
Number of shares issued	300,000
Value of shares issued	$ 15,000,000
Number of registered shares			111,000,000usl_NumberOfRegisteredUnits
Creation Baskets
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of shares per basket			50,000
Open Futures Contracts, Long \| United States
Organization, Consolidation and Presentation of Financial Statements Disclosure [Line Items]
Number of Contracts			1,349