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Financial Instruments and Concentrations of Risk (Details 2)
 12 Months Ended
Dec. 31, 2014
bbl
Dec. 31, 2013
bbl
Dec. 31, 2012
bbl
Sales [Member]      
Derivative, Nonmonetary Notional Amount, Volume 6,773,000us-gaap_DerivativeNonmonetaryNotionalAmountVolume
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SalesMember		 2,595,000us-gaap_DerivativeNonmonetaryNotionalAmountVolume
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SalesMember		 1,743,000us-gaap_DerivativeNonmonetaryNotionalAmountVolume
/ us-gaap_DerivativeByNatureAxis
= us-gaap_SalesMember
Purchases [Member]      
Derivative, Nonmonetary Notional Amount, Volume 6,477,000us-gaap_DerivativeNonmonetaryNotionalAmountVolume
/ us-gaap_DerivativeByNatureAxis
= semg_PurchasesMember		 2,575,000us-gaap_DerivativeNonmonetaryNotionalAmountVolume
/ us-gaap_DerivativeByNatureAxis
= semg_PurchasesMember		 1,636,000us-gaap_DerivativeNonmonetaryNotionalAmountVolume
/ us-gaap_DerivativeByNatureAxis
= semg_PurchasesMember