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Schedule of Investments (Parenthetical) (USD $) | 12 Months Ended
--- | ---
Dec. 31, 2014 | Dec. 31, 2013
Collateral Amount On Open Futures Contracts | 19,785,491 usl_CollateralAmountOnOpenFuturesContracts | 3,548,144 usl_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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