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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 | 166,000,000 us-gaap_LineOfCredit |  | $ 0 us-gaap_LineOfCredit |  |  |  |  |  |
Line of Credit [Member] |  |  |  |  |  |  |  |  |
Line of Credit Facility [Line Items] |  |  |  |  |  |  |  |  |
Long-term Line of Credit | 166,000,000 us-gaap_LineOfCredit / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember |  | 0 us-gaap_LineOfCredit / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember |  |  |  |  |  |
Letters of Credit Outstanding, Amount | 808,000 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember | [1] | 808,000 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember | [1] |  |  |  |  |
Line of Credit Facility, Remaining Borrowing Capacity | 1,333,192,000 us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember |  | 1,299,192,000 us-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 |  |  |  |  | 3,400,000 us-gaap_DeferredFinanceCostsGross / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember / us-gaap_LongtermDebtTypeAxis = us-gaap_RevolvingCreditFacilityMember |  |  |  |
Line of Credit Facility, Maximum Borrowing Capacity |  |  |  |  | 2,500,000,000 us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember / us-gaap_LongtermDebtTypeAxis = us-gaap_RevolvingCreditFacilityMember |  |  |  |
Line of Credit Facility, Current Borrowing Capacity |  |  |  |  | 1,500,000,000 us-gaap_LineOfCreditFacilityCurrentBorrowingCapacity / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember / us-gaap_LongtermDebtTypeAxis = us-gaap_RevolvingCreditFacilityMember |  |  |  |
Borrowing Base, Line of Credit |  |  |  |  |  | 2,400,000,000 sm_BorrowingBaseLineOfCredit / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember / us-gaap_LongtermDebtTypeAxis = us-gaap_RevolvingCreditFacilityMember | 2,200,000,000 sm_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 |  |  |  |  |  |  |  | 341,000,000 us-gaap_LineOfCredit / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember |
Letters of Credit Outstanding, Amount |  |  |  |  |  |  |  | 808,000 us-gaap_LettersOfCreditOutstandingAmount / us-gaap_DebtInstrumentAxis = us-gaap_LineOfCreditMember / us-gaap_SubsequentEventTypeAxis = us-gaap_SubsequentEventMember | [1]
Line of Credit Facility, Remaining Borrowing Capacity |  |  |  |  |  |  |  | $ 1,158,192,000 us-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.

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- Definition Value of assets pledged as collateral under line of credit agreement, net of certain company liabilities + References No definition available. + Details
Name: | sm_BorrowingBaseLineOfCredit
Namespace Prefix: | sm_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Percentage of Proved Property Secured for Credit Facility Borrowing + References No definition available. + Details
Name: | sm_PercentageofProvedPropertySecuredforCreditFacilityBorrowing
Namespace Prefix: | sm_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Percentage points added to the reference rate to compute the variable rate on the debt instrument. + References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition For an unclassified balance sheet, the amount as of the balance sheet date of capitalized costs associated with the issuance of debt instruments (for example, legal, accounting, underwriting, printing, and registration costs) that will be charged against earnings over the life of the debt instruments to which such costs pertain. Such amount is before the consideration of accumulated amortization. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 835 -SubTopic 30 -Section 45 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28555-108399 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | debit
Period Type: | instant

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- Definition The total amount of the contingent obligation under letters of credit outstanding as of the reporting date. + References No definition available. + Details
Name: | us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition The carrying value as of the balance sheet date of the current and noncurrent portions of long-term obligations drawn from a line of credit, which is a bank's commitment to make loans up to a specific amount. Examples of items that might be included in the application of this element may consist of letters of credit, standby letters of credit, and revolving credit arrangements, under which borrowings can be made up to a maximum amount as of any point in time conditional on satisfaction of specified terms before, as of and after the date of drawdowns on the line. Includes short-term obligations that would normally be classified as current liabilities but for which (a) postbalance sheet date issuance of a long term obligation to refinance the short term obligation on a long term basis, or (b) the enterprise has entered into a financing agreement that clearly permits the enterprise to refinance the short-term obligation on a long term basis and the following conditions are met (1) the agreement does not expire within 1 year and is not cancelable by the lender except for violation of an objectively determinable provision, (2) no violation exists at the BS date, and (3) the lender has entered into the financing agreement is expected to be financially capable of honoring the agreement. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03.16) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 20, 22 -Article 5 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03.16) -URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910 + Details
Name: | us-gaap_LineOfCredit
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of current borrowing capacity under the credit facility considering any current restrictions on the amount that could be borrowed (for example, borrowings may be limited by the amount of current assets), but without considering any amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Description of the interest rate for the amounts borrowed under the credit facility, including the terms and the method for determining the interest rate (for example, fixed or variable, LIBOR plus a percentage, increasing rate, timing of interest rate resets, remarketing provisions). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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- Details
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- Definition Maximum borrowing capacity under the credit facility without consideration of any current restrictions on the amount that could be borrowed or the amounts currently outstanding under the facility. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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Balance Type: | credit
Period Type: | instant

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- Definition Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.19(b),22(b)) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Section 02 -Paragraph 19, 22 -Article 5 + Details
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Period Type: | instant

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- Definition The fee, expressed as a percentage of the line of credit facility, for available but unused credit capacity under the credit facility. + References No definition available. + Details
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- Details
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- Details
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Namespace Prefix: |
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Period Type: |

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- Details
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