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Fair Value of Financial Instruments (Effect of Derivative Instruments on Condensed Statements of Operations) (Details) (USD $) | 12 Months Ended
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Dec. 31, 2014 | Dec. 31, 2013 | Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Change in Unrealized Gain or (Loss) on Derivatives Recognized in Income | $ (18,738,510)us-gaap_UnrealizedGainLossOnDerivativesAndCommodityContracts | $ 85,540 us-gaap_UnrealizedGainLossOnDerivativesAndCommodityContracts | $ (2,445,210)us-gaap_UnrealizedGainLossOnDerivativesAndCommodityContracts
Commodity Contracts | Futures | Realized gain on closed positions |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Realized Gain on Derivatives Recognized in Income | 402,190 us-gaap_GainLossOnSaleOfDerivatives / us-gaap_BalanceSheetLocationAxis = usl_ClosedPositionsMember / us-gaap_DerivativeByNatureAxis = us-gaap_FutureMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember | 6,302,760 us-gaap_GainLossOnSaleOfDerivatives / us-gaap_BalanceSheetLocationAxis = usl_ClosedPositionsMember / us-gaap_DerivativeByNatureAxis = us-gaap_FutureMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember | (6,954,520)us-gaap_GainLossOnSaleOfDerivatives / us-gaap_BalanceSheetLocationAxis = usl_ClosedPositionsMember / us-gaap_DerivativeByNatureAxis = us-gaap_FutureMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember
Commodity Contracts | Futures | Change in unrealized gain (loss) on open positions |  |  |
Derivative Instruments, Gain (Loss) [Line Items] |  |  |
Change in Unrealized Gain or (Loss) on Derivatives Recognized in Income | $ (18,738,510)us-gaap_UnrealizedGainLossOnDerivativesAndCommodityContracts / us-gaap_BalanceSheetLocationAxis = usl_OpenPositionMember / us-gaap_DerivativeByNatureAxis = us-gaap_FutureMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember | $ 85,540 us-gaap_UnrealizedGainLossOnDerivativesAndCommodityContracts / us-gaap_BalanceSheetLocationAxis = usl_OpenPositionMember / us-gaap_DerivativeByNatureAxis = us-gaap_FutureMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember | $ (2,445,210)us-gaap_UnrealizedGainLossOnDerivativesAndCommodityContracts / us-gaap_BalanceSheetLocationAxis = usl_OpenPositionMember / us-gaap_DerivativeByNatureAxis = us-gaap_FutureMember / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_CommodityContractMember

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- Definition The difference between the book value and the sale price of options, swaps, futures, forward contracts, and other derivative instruments. This element refers to the gain (loss) included in earnings. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 225 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-04.13(h)) -URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882 + Details
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- Definition The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of open derivatives, commodity, or energy contracts, held at each balance sheet date, that was included in earnings for the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=31042434&loc=d3e3602-108585 + Details
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