Collateral Coverage Ratio

No definition available.

Name:	swn_CollateralCoverageRatio
Namespace Prefix:	swn_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM format.

+ References

No definition available.

+ Details

Name:	swn_DebtInstrumentMaturityMonthAndYear
Namespace Prefix:	swn_
Data Type:	xbrli:gYearMonthItemType
Balance Type:	na
Period Type:	duration

- Definition

Total debt divided by EBITDAX.

+ References

No definition available.

+ Details

Name:	swn_LeverageRatio
Namespace Prefix:	swn_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Liquidity Requirement

+ References

No definition available.

+ Details

Name:	swn_LiquidityRequirement
Namespace Prefix:	swn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

EBITDAX divided by consolidated interest expense.

+ References

No definition available.

+ Details

Name:	swn_MinimumInterestCoverageRatio
Namespace Prefix:	swn_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	duration

- Definition

Net cash proceeds from asset sales allowed to be retained.

+ References

No definition available.

+ Details

Name:	swn_NetCashProceedsFromAssetSalesAllowedToBeRetained
Namespace Prefix:	swn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, before unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt.

+ 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(22))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: 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

Reference 3: 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

+ Details

Name:	us-gaap_DebtInstrumentCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- 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

+ Details

Name:	us-gaap_LineOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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

+ Details

Name:	us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of cash inflow from contractual arrangement with the lender, including but not limited to, letter of credit, standby letter of credit and revolving credit arrangements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 14
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3255-108585

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(f))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

+ Details

Name:	us-gaap_ProceedsFromLinesOfCredit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

The cash outflow for debt initially having maturity due after one year or beyond the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3291-108585

+ Details

Name:	us-gaap_RepaymentsOfLongTermDebt
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_LongTermDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=swn_CreditFacilityOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=swn_CreditFacilityTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=us-gaap_LetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=swn_RevolvingCreditFacilityAndTermLoanFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_EurodollarMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_BaseRateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=swn_TermLoanThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt (2016 Credit Facility - Narrative) (Details)		1 Months Ended		3 Months Ended		6 Months Ended	12 Months Ended
Debt (2016 Credit Facility - Narrative) (Details)	Apr. 26, 2018 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2016 USD ($)	Mar. 31, 2018 USD ($)	Jun. 30, 2017	Dec. 31, 2018	Dec. 31, 2017 USD ($)	Dec. 31, 2013 USD ($)
Long-term Debt [Member]
Debt Instrument [Line Items]
Debt instrument				$ 4,433,000,000			$ 4,433,000,000
Long-term Debt [Member] \| 2013 Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 66,000,000					$ 2,000,000,000
Line of credit, amount borrowed				0
Long-term Debt [Member] \| 2016 Credit Facility [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 743,000,000	743,000,000
Line of credit, amount borrowed				0
Long-term Debt [Member] \| Letter of Credit [Member]
Debt Instrument [Line Items]
Amount outstanding	$ 323,000,000			$ 323,000,000
Revolving Credit Facility And Term Loan Facility [Member]
Debt Instrument [Line Items]
Maturity date, year and month			2020-12
Revolving Credit Facility And Term Loan Facility [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Maximum borrowing capacity			$ 1,934,000,000
Leverage ratio		4.00		5.00
Net cash proceeds from asset sales allowed to be retained		$ 500,000,000
Revolving Credit Facility And Term Loan Facility [Member] \| Long-term Debt [Member] \| Scenario, Forecast [Member]
Debt Instrument [Line Items]
Leverage ratio						4.50
Revolving Credit Facility And Term Loan Facility [Member] \| Long-term Debt [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Minimum interest coverage ratio					2.00
Liquidity requirement				$ 300,000,000
Revolving Credit Facility And Term Loan Facility [Member] \| Long-term Debt [Member] \| Eurodollar [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Basis points				1.75%
Revolving Credit Facility And Term Loan Facility [Member] \| Long-term Debt [Member] \| Eurodollar [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Basis points				2.50%
Revolving Credit Facility And Term Loan Facility [Member] \| Long-term Debt [Member] \| Base Rate [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Basis points				0.75%
Revolving Credit Facility And Term Loan Facility [Member] \| Long-term Debt [Member] \| Base Rate [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Basis points				1.50%
Revolving Credit Facility And Term Loan Facility [Member] \| Long-term Debt [Member] \| Over LIBOR [Member]
Debt Instrument [Line Items]
Basis points				2.50%
2016 Term Loan due December 2020 [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Debt instrument			$ 1,191,000,000	$ 1,191,000,000			$ 1,191,000,000
Maturity date, year and month				2020-12			2020-12
Collateral coverage ratio				1.50
Subsequent Event [Member] \| 2016 Term Loan due December 2020 [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Repayments of long-term debt	$ 1,191,000,000