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Long-term Debt Revolving Credit Facility (Details) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 10, 2014
Oct. 06, 2014
Sep. 06, 2013
Feb. 18, 2015
Line of Credit Facility [Line Items]            
Long-term Line of Credit 166,000,000us-gaap_LineOfCredit $ 0us-gaap_LineOfCredit        
Line of Credit [Member]            
Line of Credit Facility [Line Items]            
Long-term Line of Credit 166,000,000us-gaap_LineOfCredit
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember		 0us-gaap_LineOfCredit
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember		        
Letters of Credit Outstanding, Amount 808,000us-gaap_LettersOfCreditOutstandingAmount
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember		 [1] 808,000us-gaap_LettersOfCreditOutstandingAmount
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember		 [1]        
Line of Credit Facility, Remaining Borrowing Capacity 1,333,192,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember		 1,299,192,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember		        
Line of Credit [Member] | Borrowing Base Utilization of 25 Percent [Member]            
Line of Credit Facility [Line Items]            
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.30%us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ sm_DebtInstrumentBorrowingBaseUtilizationPercentageAxis
= sm_BorrowingBaseUtilizationOf25PercentMember		          
Line of Credit [Member] | Borrowing Base Utilization Of More Than 25 Percent But Less Than 50 Percent [Member]            
Line of Credit Facility [Line Items]            
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.30%us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ sm_DebtInstrumentBorrowingBaseUtilizationPercentageAxis
= sm_BorrowingBaseUtilizationOfMoreThan25PercentButLessThan50PercentMember		          
Line of Credit [Member] | Borrowing Base Utilization Of More Than 50 Percent But Less Than 75 Percent [Member]            
Line of Credit Facility [Line Items]            
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.35%us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ sm_DebtInstrumentBorrowingBaseUtilizationPercentageAxis
= sm_BorrowingBaseUtilizationOfMoreThan50PercentButLessThan75PercentMember		          
Line of Credit [Member] | Borrowing Base Utilization Of More Than 75 Percent But Less Than 90 Percent [Member]            
Line of Credit Facility [Line Items]            
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.375%us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ sm_DebtInstrumentBorrowingBaseUtilizationPercentageAxis
= sm_BorrowingBaseUtilizationOfMoreThan75PercentButLessThan90PercentMember		          
Line of Credit [Member] | Borrowing Base Utilization Of More Than 90 Percent [Member]            
Line of Credit Facility [Line Items]            
Line of Credit Facility, Unused Capacity, Commitment Fee Percentage 0.375%us-gaap_LineOfCreditFacilityUnusedCapacityCommitmentFeePercentage
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ sm_DebtInstrumentBorrowingBaseUtilizationPercentageAxis
= sm_BorrowingBaseUtilizationOfMoreThan90PercentMember		          
Line of Credit [Member] | Revolving Credit Facility [Member]            
Line of Credit Facility [Line Items]            
Deferred Finance Costs, Gross     3,400,000us-gaap_DeferredFinanceCostsGross
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_RevolvingCreditFacilityMember		      
Line of Credit Facility, Maximum Borrowing Capacity     2,500,000,000us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_RevolvingCreditFacilityMember		      
Line of Credit Facility, Current Borrowing Capacity     1,500,000,000us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_RevolvingCreditFacilityMember		      
Borrowing Base, Line of Credit       2,400,000,000sm_BorrowingBaseLineOfCredit
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_RevolvingCreditFacilityMember		 2,200,000,000sm_BorrowingBaseLineOfCredit
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_RevolvingCreditFacilityMember		  
Percentage of Proved Oil and Gas Properties Secured for Credit Facility Borrowing 75.00%sm_PercentageofProvedPropertySecuredforCreditFacilityBorrowing
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_RevolvingCreditFacilityMember		          
Eurodollar [Member] | Line of Credit [Member] | Borrowing Base Utilization of 25 Percent [Member]            
Line of Credit Facility [Line Items]            
Debt Instrument, Basis Spread on Variable Rate 1.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ sm_DebtInstrumentBorrowingBaseUtilizationPercentageAxis
= sm_BorrowingBaseUtilizationOf25PercentMember
/ sm_DebtInstrumentVariableRateLoansAxis
= us-gaap_EurodollarMember		          
Eurodollar [Member] | Line of Credit [Member] | Borrowing Base Utilization Of More Than 25 Percent But Less Than 50 Percent [Member]            
Line of Credit Facility [Line Items]            
Debt Instrument, Basis Spread on Variable Rate 1.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ sm_DebtInstrumentBorrowingBaseUtilizationPercentageAxis
= sm_BorrowingBaseUtilizationOfMoreThan25PercentButLessThan50PercentMember
/ sm_DebtInstrumentVariableRateLoansAxis
= us-gaap_EurodollarMember		          
Eurodollar [Member] | Line of Credit [Member] | Borrowing Base Utilization Of More Than 50 Percent But Less Than 75 Percent [Member]            
Line of Credit Facility [Line Items]            
Debt Instrument, Basis Spread on Variable Rate 1.75%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ sm_DebtInstrumentBorrowingBaseUtilizationPercentageAxis
= sm_BorrowingBaseUtilizationOfMoreThan50PercentButLessThan75PercentMember
/ sm_DebtInstrumentVariableRateLoansAxis
= us-gaap_EurodollarMember		          
Eurodollar [Member] | Line of Credit [Member] | Borrowing Base Utilization Of More Than 75 Percent But Less Than 90 Percent [Member]            
Line of Credit Facility [Line Items]            
Debt Instrument, Basis Spread on Variable Rate 2.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ sm_DebtInstrumentBorrowingBaseUtilizationPercentageAxis
= sm_BorrowingBaseUtilizationOfMoreThan75PercentButLessThan90PercentMember
/ sm_DebtInstrumentVariableRateLoansAxis
= us-gaap_EurodollarMember		          
Eurodollar [Member] | Line of Credit [Member] | Borrowing Base Utilization Of More Than 90 Percent [Member]            
Line of Credit Facility [Line Items]            
Debt Instrument, Basis Spread on Variable Rate 2.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ sm_DebtInstrumentBorrowingBaseUtilizationPercentageAxis
= sm_BorrowingBaseUtilizationOfMoreThan90PercentMember
/ sm_DebtInstrumentVariableRateLoansAxis
= us-gaap_EurodollarMember		          
Eurodollar [Member] | Line of Credit [Member] | Revolving Credit Facility [Member]            
Line of Credit Facility [Line Items]            
Line of Credit Facility, Interest Rate Description London Interbank Offered Rate plus the applicable margin from the utilization table          
Debt Instrument Variable Rate, Alternative Base Rate, And Swingline Loans [Member] | Line of Credit [Member] | Borrowing Base Utilization of 25 Percent [Member]            
Line of Credit Facility [Line Items]            
Debt Instrument, Basis Spread on Variable Rate 0.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ sm_DebtInstrumentBorrowingBaseUtilizationPercentageAxis
= sm_BorrowingBaseUtilizationOf25PercentMember
/ sm_DebtInstrumentVariableRateLoansAxis
= sm_DebtInstrumentVariableRateAlternativeBaseRateAndSwinglineLoansMember		          
Debt Instrument Variable Rate, Alternative Base Rate, And Swingline Loans [Member] | Line of Credit [Member] | Borrowing Base Utilization Of More Than 25 Percent But Less Than 50 Percent [Member]            
Line of Credit Facility [Line Items]            
Debt Instrument, Basis Spread on Variable Rate 0.50%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ sm_DebtInstrumentBorrowingBaseUtilizationPercentageAxis
= sm_BorrowingBaseUtilizationOfMoreThan25PercentButLessThan50PercentMember
/ sm_DebtInstrumentVariableRateLoansAxis
= sm_DebtInstrumentVariableRateAlternativeBaseRateAndSwinglineLoansMember		          
Debt Instrument Variable Rate, Alternative Base Rate, And Swingline Loans [Member] | Line of Credit [Member] | Borrowing Base Utilization Of More Than 50 Percent But Less Than 75 Percent [Member]            
Line of Credit Facility [Line Items]            
Debt Instrument, Basis Spread on Variable Rate 0.75%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ sm_DebtInstrumentBorrowingBaseUtilizationPercentageAxis
= sm_BorrowingBaseUtilizationOfMoreThan50PercentButLessThan75PercentMember
/ sm_DebtInstrumentVariableRateLoansAxis
= sm_DebtInstrumentVariableRateAlternativeBaseRateAndSwinglineLoansMember		          
Debt Instrument Variable Rate, Alternative Base Rate, And Swingline Loans [Member] | Line of Credit [Member] | Borrowing Base Utilization Of More Than 75 Percent But Less Than 90 Percent [Member]            
Line of Credit Facility [Line Items]            
Debt Instrument, Basis Spread on Variable Rate 1.00%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ sm_DebtInstrumentBorrowingBaseUtilizationPercentageAxis
= sm_BorrowingBaseUtilizationOfMoreThan75PercentButLessThan90PercentMember
/ sm_DebtInstrumentVariableRateLoansAxis
= sm_DebtInstrumentVariableRateAlternativeBaseRateAndSwinglineLoansMember		          
Debt Instrument Variable Rate, Alternative Base Rate, And Swingline Loans [Member] | Line of Credit [Member] | Borrowing Base Utilization Of More Than 90 Percent [Member]            
Line of Credit Facility [Line Items]            
Debt Instrument, Basis Spread on Variable Rate 1.25%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ sm_DebtInstrumentBorrowingBaseUtilizationPercentageAxis
= sm_BorrowingBaseUtilizationOfMoreThan90PercentMember
/ sm_DebtInstrumentVariableRateLoansAxis
= sm_DebtInstrumentVariableRateAlternativeBaseRateAndSwinglineLoansMember		          
Debt Instrument Variable Rate, Alternative Base Rate, And Swingline Loans [Member] | Line of Credit [Member] | Revolving Credit Facility [Member]            
Line of Credit Facility [Line Items]            
Line of Credit Facility, Interest Rate Description Prime plus the applicable margin from the utilization table          
Subsequent Event [Member] | Line of Credit [Member]            
Line of Credit Facility [Line Items]            
Long-term Line of Credit           341,000,000us-gaap_LineOfCredit
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
Letters of Credit Outstanding, Amount           808,000us-gaap_LettersOfCreditOutstandingAmount
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember		 [1]
Line of Credit Facility, Remaining Borrowing Capacity           $ 1,158,192,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_DebtInstrumentAxis
= us-gaap_LineOfCreditMember
/ us-gaap_SubsequentEventTypeAxis
= us-gaap_SubsequentEventMember
[1] (1) Letters of credit reduce the amount available under the credit facility on a dollar-for-dollar basis.