Increase in percentage points added to the reference rate to compute the variable rate on the debt instrument.

No definition available.

Name:	swn_DebtInstrumentIncreaseInBasisSpreadOnVariableRate
Namespace Prefix:	swn_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Long Term Debt, Current Maturities, Gross

+ References

No definition available.

+ Details

Name:	swn_LongTermDebtCurrentMaturitiesGross
Namespace Prefix:	swn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Long Term Debt, Excluding Current Maturities, Gross

+ References

No definition available.

+ Details

Name:	swn_LongTermDebtExcludingCurrentMaturitiesGross
Namespace Prefix:	swn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- 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/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(1)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-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://fasb.org/us-gaap/role/ref/legacyRef
-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 4: http://fasb.org/us-gaap/role/ref/legacyRef
-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

The average effective interest rate during the reporting period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28551-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Contractual interest rate for funds borrowed, under the debt agreement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.22(a)(1))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentInterestRateStatedPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
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

Amount, after accumulated amortization, of debt discount.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=114775744&loc=d3e28541-108399

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=114775985&loc=d3e28878-108400

+ Details

Name:	us-gaap_DebtInstrumentUnamortizedDiscount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=109500613&loc=SL6031897-161870

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(16))
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-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 4: http://fasb.org/us-gaap/role/ref/legacyRef
-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

+ Details

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

- Definition

Amount, after unamortized (discount) premium and debt issuance costs, of long-term debt, classified as current. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.19,20)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Amount after unamortized (discount) premium and debt issuance costs of long-term debt classified as noncurrent and excluding amounts to be repaid within one year or the normal operating cycle, if longer. Includes, but not limited to, notes payable, bonds payable, debentures, mortgage loans and commercial paper. Excludes capital lease obligations.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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

+ Details

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

- Definition

The remaining balance of debt issuance expenses that were capitalized and are being amortized against income over the lives of the respective bond issues. This does not include the amounts capitalized as part of the cost of the utility plant or asset.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_UnamortizedDebtIssuanceExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=swn_A2018RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=swn_A2018TermLoanFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

Debt (Components of Debt) (Details) - USD ($) $ in Millions	1 Months Ended		6 Months Ended	12 Months Ended
Debt (Components of Debt) (Details) - USD ($) $ in Millions	Jul. 31, 2016	Feb. 29, 2016	Jun. 30, 2019	Dec. 31, 2018	Jul. 31, 2018	Jan. 31, 2015
Debt Instrument [Line Items]
Debt Instrument, long-term debt			$ 2,290
Debt Instrument			2,342	$ 2,342
Unamortized Issuance Expense			(22)	(23)
Unamortized Debt Discount			(1)	(1)
Total, current portion of long-term debt			52	0
Total, long-term debt			2,267	2,318
Total debt			2,319	2,318
Unamortized issuance expense			22	23
Line of Credit \| 2018 revolving credit facility, due April 2023
Debt Instrument [Line Items]
Debt Instrument, long-term debt			0
Unamortized Issuance Expense			0
Unamortized Debt Discount			0
Total, long-term debt			$ 0
Variable interest rate			3.88%
Unamortized issuance expense			$ 0
Line of Credit \| 2018 term loan facility, due April 2023
Debt Instrument [Line Items]
Debt Instrument				0
Unamortized Issuance Expense				0
Unamortized Debt Discount				0
Total debt				$ 0
Variable interest rate				3.92%
Unamortized issuance expense				$ 0
Senior Notes
Debt Instrument [Line Items]
Debt Instrument, current portion of long-term debt			52
Unamortized Issuance Expense			0
Unamortized Debt Discount			0
Total, current portion of long-term debt			52
Unamortized issuance expense			0
Senior Notes \| 4.05% Senior Notes due January 2020
Debt Instrument [Line Items]
Debt Instrument, current portion of long-term debt			52
Debt Instrument				52
Unamortized Issuance Expense			0	0
Unamortized Debt Discount			0	0
Total, current portion of long-term debt			$ 52
Total debt				52
Stated interest rate			4.05%
Unamortized issuance expense			$ 0	0
Senior Notes \| 4.10% Senior Notes due March 2022
Debt Instrument [Line Items]
Debt Instrument, long-term debt			213
Debt Instrument				213
Unamortized Issuance Expense			(1)	(1)
Unamortized Debt Discount			0	0
Total, long-term debt			$ 212
Total debt				212
Stated interest rate			4.10%
Unamortized issuance expense			$ 1	1
Senior Notes \| 4.95% Senior Notes due January 2025
Debt Instrument [Line Items]
Debt Instrument, long-term debt			927
Debt Instrument				927
Unamortized Issuance Expense			(7)	(7)
Unamortized Debt Discount			(1)	(1)
Total, long-term debt			$ 919
Total debt				919
Stated interest rate			4.95%
Unamortized issuance expense			$ 7	7
Senior Notes \| 7.50% Senior Notes due April 2026
Debt Instrument [Line Items]
Debt Instrument, long-term debt			650
Debt Instrument				650
Unamortized Issuance Expense			(8)	(8)
Unamortized Debt Discount			0	0
Total, long-term debt			$ 642
Total debt				642
Stated interest rate			7.50%
Unamortized issuance expense			$ 8	8
Senior Notes \| 7.75% Senior Notes due October 2027
Debt Instrument [Line Items]
Debt Instrument, long-term debt			500
Debt Instrument				500
Unamortized Issuance Expense			(6)	(7)
Unamortized Debt Discount			0	0
Total, long-term debt			$ 494
Total debt				493
Stated interest rate			7.75%
Unamortized issuance expense			$ 6	7
Long-term debt \| 4.05% Senior Notes due January 2020
Debt Instrument [Line Items]
Stated interest rate	5.80%		6.05%		5.30%	4.05%
Long-term debt \| 4.95% Senior Notes due January 2025
Debt Instrument [Line Items]
Stated interest rate	6.70%		6.95%		6.20%	4.95%
LIBOR \| Senior Notes
Debt Instrument [Line Items]
Increase in basis spread	1.75%	1.75%
Other long-term assets \| Line of Credit \| 2018 revolving credit facility, due April 2023
Debt Instrument [Line Items]
Unamortized Issuance Expense			$ (10)	(11)
Unamortized issuance expense			$ 10	$ 11