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Schedule of Investments (Parenthetical) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013
Collateral Amount On Open Futures Contracts	19,785,491usl_CollateralAmountOnOpenFuturesContracts	3,548,144usl_CollateralAmountOnOpenFuturesContracts
U.S. Treasury Bill, 0.05%, 1/15/2015 | US Treasury Securities | Cash Equivalents
Interest rate	0.05%us-gaap_DebtInstrumentInterestRateStatedPercentage / invest_InvestmentAxis = us-gaap_CashEquivalentsMember / invest_InvestmentSecuritiesClassAxis = us-gaap_USTreasurySecuritiesMember / invest_InvestmentSecuritiesSeriesAxis = us-gaap_USTreasuryBillSecuritiesMember
Expiration date	1/15/2015
U.S. Treasury Bill 0.10%, 6/25/2015 | US Treasury Securities | Cash Equivalents
Interest rate	0.10%us-gaap_DebtInstrumentInterestRateStatedPercentage / invest_InvestmentAxis = us-gaap_CashEquivalentsMember / invest_InvestmentSecuritiesClassAxis = us-gaap_USTreasurySecuritiesMember / invest_InvestmentSecuritiesSeriesAxis = usl_UsTreasuryBillSecurities1Member
Expiration date	6/25/2015
United States | NYMEX WTI Crude Oil Futures CL May 2015 contracts, expiring April 2015 | Open Futures Contracts, Long
Expiration date	April 2015
United States | NYMEX WTI Crude Oil Futures CL June 2015 contracts, expiring May 2015 | Open Futures Contracts, Long
Expiration date	May 2015
United States | NYMEX WTI Crude Oil Futures CL July 2015 contracts, expiring June 2015 | Open Futures Contracts, Long
Expiration date	June 2015
United States | NYMEX WTI Crude Oil Futures CL February 2015 contracts, expiring January 2015 | Open Futures Contracts, Long
Expiration date	January 2015
United States | NYMEX WTI Crude Oil Futures CL March 2015 contracts, expiring February 2015 | Open Futures Contracts, Long
Expiration date	February 2015
United States | NYMEX WTI Crude Oil Futures CL April 2015 contracts, expiring March 2015 | Open Futures Contracts, Long
Expiration date	March 2015
United States | NYMEX WTI Crude Oil Futures CL August 2015 contracts, expiring July 2015 | Open Futures Contracts, Long
Expiration date	July 2015
United States | NYMEX WTI Crude Oil Futures CL September 2015 contracts, expiring August 2015 | Open Futures Contracts, Long
Expiration date	August 2015
United States | NYMEX WTI Crude Oil Futures CL November 2015 contracts, expiring October 2015 | Open Futures Contracts, Long
Expiration date	October 2015
United States | NYMEX WTI Crude Oil Futures CL October 2015 contracts, expiring September 2015 | Open Futures Contracts, Long
Expiration date	September 2015
United States | NYMEX WTI Crude Oil Futures CL December 2015 contracts, expiring November 2015 | Open Futures Contracts, Long
Expiration date	November 2015
United States | NYMEX WTI Crude Oil Futures CL January 2016 contracts, expiring December 2015 | Open Futures Contracts, Long
Expiration date	December 2015
United States | U.S. Treasury Bill 0.7%, 3/27/2014 | Open Futures Contracts, Long
Interest rate		0.70%us-gaap_DebtInstrumentInterestRateStatedPercentage / invest_InvestmentGeographicRegionAxis = country_US / invest_InvestmentSecuritiesSeriesAxis = usl_UsTreasuryBillSecurities3Member / us-gaap_InvestmentTypeAxis = usl_OpenFuturesContractsLongMember
Expiration date		3/27/2014
United States | NYMEX WTI Crude Oil Futures CL February 2014 contracts, expiring January 2014 | Open Futures Contracts, Long
Expiration date		January 2014
United States | NYMEX WTI Crude Oil Futures CL March 2014 contracts, expiring February 2014 | Open Futures Contracts, Long
Expiration date		February 2014
United States | NYMEX WTI Crude Oil Futures CL April 2014 contracts, expiring March 2014 | Open Futures Contracts, Long
Expiration date		March 2014
United States | NYMEX WTI Crude Oil Futures CL May 2014 contracts, expiring April 2014 | Open Futures Contracts, Long
Expiration date		April 2014
United States | NYMEX WTI Crude Oil Futures CL June 2014 contracts, expiring May 2014 | Open Futures Contracts, Long
Expiration date		May 2014
United States | NYMEX WTI Crude Oil Futures CL July 2014 contracts, expiring June 2014 | Open Futures Contracts, Long
Expiration date		June 2014
United States | NYMEX WTI Crude Oil Futures CL August 2014 contracts, expiring July 2014 | Open Futures Contracts, Long
Expiration date		July 2014
United States | NYMEX WTI Crude Oil Futures CL September 2014 contracts, expiring August 2014 | Open Futures Contracts, Long
Expiration date		August 2014
United States | NYMEX WTI Crude Oil Futures CL October 2014 contracts, expiring September 2014 | Open Futures Contracts, Long
Expiration date		September 2014
United States | NYMEX WTI Crude Oil Futures CL November 2014 contracts, expiring October 2014 | Open Futures Contracts, Long
Expiration date		October 2014
United States | NYMEX WTI Crude Oil Futures CL December 2014 contracts, expiring November 2014 | Open Futures Contracts, Long
Expiration date		November 2014
United States | NYMEX WTI Crude Oil Futures CL January 2015 contracts, expiring December 2014 | Open Futures Contracts, Long
Expiration date		December 2014

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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Reference 1: http://www.xbrl.org/2003/role/presentationRef

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Reference 2: http://www.xbrl.org/2003/role/presentationRef

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Reference 3: http://www.xbrl.org/2003/role/presentationRef

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