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Statements of Changes in Partners' Capital (USD $)
Total
General Partner [Member]
Limited Partners
Beginning Balances at Dec. 31, 2014 $ 37,232,925us-gaap_PartnersCapital $ 0us-gaap_PartnersCapital
/ us-gaap_PartnerCapitalComponentsAxis
= us-gaap_GeneralPartnerMember		 $ 37,232,925us-gaap_PartnersCapital
/ us-gaap_PartnerCapitalComponentsAxis
= us-gaap_LimitedPartnerMember
Net asset value, beginning of period at Dec. 31, 2014 $ 26.59usl_NetAssetValuePerUnit    
Addition of 1,600,000 partnership shares 39,864,429us-gaap_PartnersCapitalAccountContributions 0us-gaap_PartnersCapitalAccountContributions
/ us-gaap_PartnerCapitalComponentsAxis
= us-gaap_GeneralPartnerMember		 39,864,429us-gaap_PartnersCapitalAccountContributions
/ us-gaap_PartnerCapitalComponentsAxis
= us-gaap_LimitedPartnerMember
Net income (loss) (5,217,256)us-gaap_NetIncomeLoss 0us-gaap_NetIncomeLoss
/ us-gaap_PartnerCapitalComponentsAxis
= us-gaap_GeneralPartnerMember		 (5,217,256)us-gaap_NetIncomeLoss
/ us-gaap_PartnerCapitalComponentsAxis
= us-gaap_LimitedPartnerMember
Ending Balances at Mar. 31, 2015 $ 71,880,098us-gaap_PartnersCapital $ 0us-gaap_PartnersCapital
/ us-gaap_PartnerCapitalComponentsAxis
= us-gaap_GeneralPartnerMember		 $ 71,880,098us-gaap_PartnersCapital
/ us-gaap_PartnerCapitalComponentsAxis
= us-gaap_LimitedPartnerMember
Net asset value, end of period at Mar. 31, 2015 $ 23.96usl_NetAssetValuePerUnit