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FAIR VALUE OF FINANCIAL INSTRUMENTS (Effect of Derivative Instruments on Condensed Statements of Operations) (Details) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Derivative Instruments, Gain (Loss) [Line Items]      
Realized Gain or (Loss) on Derivatives Recognized in Income $ 2,205,950us-gaap_GainLossOnSaleOfInvestments $ 2,395,300us-gaap_GainLossOnSaleOfInvestments $ (12,161,650)us-gaap_GainLossOnSaleOfInvestments
Change in Unrealized Gain (Loss) on Derivatives Recognized in Income (6,009,940)us-gaap_UnrealizedGainLossOnDerivativesAndCommodityContracts 1,996,630us-gaap_UnrealizedGainLossOnDerivativesAndCommodityContracts 7,676,740us-gaap_UnrealizedGainLossOnDerivativesAndCommodityContracts
Commodity Contracts [Member] | Futures [Member] | Realized gain on closed positions      
Derivative Instruments, Gain (Loss) [Line Items]      
Realized Gain or (Loss) on Derivatives Recognized in Income 2,205,950us-gaap_GainLossOnSaleOfInvestments
/ us-gaap_BalanceSheetLocationAxis
= unl_ClosedPositionsMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_FutureMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
2,395,300us-gaap_GainLossOnSaleOfInvestments
/ us-gaap_BalanceSheetLocationAxis
= unl_ClosedPositionsMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_FutureMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
(12,161,650)us-gaap_GainLossOnSaleOfInvestments
/ us-gaap_BalanceSheetLocationAxis
= unl_ClosedPositionsMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_FutureMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
Commodity Contracts [Member] | Futures [Member] | Change in unrealized gain on open positions      
Derivative Instruments, Gain (Loss) [Line Items]      
Change in Unrealized Gain (Loss) on Derivatives Recognized in Income $ (6,009,940)us-gaap_UnrealizedGainLossOnDerivativesAndCommodityContracts
/ us-gaap_BalanceSheetLocationAxis
= unl_OpenPositionMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_FutureMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
$ 1,996,630us-gaap_UnrealizedGainLossOnDerivativesAndCommodityContracts
/ us-gaap_BalanceSheetLocationAxis
= unl_OpenPositionMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_FutureMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember
$ 7,676,740us-gaap_UnrealizedGainLossOnDerivativesAndCommodityContracts
/ us-gaap_BalanceSheetLocationAxis
= unl_OpenPositionMember
/ us-gaap_DerivativeByNatureAxis
= us-gaap_FutureMember
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_CommodityContractMember