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

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

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

Date the credit facility terminates, in CCYY-MM-DD format.

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

+ Details

Name:	us-gaap_LineOfCreditFacilityExpirationDate1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- 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 of noncurrent assets classified as other.

+ 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_OtherAssetsNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 15
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3291-108585

+ Details

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

- 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_LongTermDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditFacilityAxis=swn_CreditFacilityFourMember
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_DebtInstrumentAxis=swn_TermLoanThreeMember
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:	srt_RangeAxis=srt_MaximumMember
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_SeniorNotesMember
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_StatementScenarioAxis=us-gaap_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Debt (Components of Debt) (Details) - USD ($) $ in Millions		1 Months Ended			3 Months Ended	9 Months Ended		12 Months Ended
Debt (Components of Debt) (Details) - USD ($) $ in Millions	Apr. 26, 2018	Sep. 30, 2017	Jul. 31, 2016	Jan. 31, 2015	Dec. 31, 2018	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Jul. 31, 2018	Dec. 31, 2016	Jun. 30, 2016
Debt Instrument [Line Items]
Other long-term assets						$ 194		$ 240
Repayments of long-term debt						1,191	$ 1,139
Long-term Debt [Member]
Debt Instrument [Line Items]
Debt Instrument						3,602		4,433
Unamortized Issuance Expense						(29)		(40)
Unamortized Debt Discount						(1)		(2)
Total						3,572		4,391
Long-term Debt [Member] \| 2018 Credit Facility [Member]
Debt Instrument [Line Items]
Debt Instrument						360
Unamortized Issuance Expense						(12)
Total						$ 360
Variable interest rate						3.59%
Credit facility, maturity date	Apr. 26, 2023					Apr. 26, 2023
Long-term Debt [Member] \| 2016 Credit Facility [Member]
Debt Instrument [Line Items]
Unamortized Issuance Expense	$ (4)					$ (4)
Long-term Debt [Member] \| 2016 Term Loan due December 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument	1,191							1,191			$ 1,191
Unamortized Issuance Expense								(8)
Total								$ 1,183
Maturity date, year and month						2020-12
Repayments of long-term debt	$ 1,191
Long-term Debt [Member] \| 2016 Term Loan due December 2020 [Member] \| 2016 Credit Facility [Member]
Debt Instrument [Line Items]
Variable interest rate								3.98%
Credit facility, maturity date								Dec. 01, 2020
Long-term Debt [Member] \| 4.05% Senior Notes due January 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument						$ 92		$ 92
Total						$ 92		$ 92
Stated interest rate			5.80%	4.05%		4.05%		4.05%	5.30%
Maturity date, year and month				2020-01		2020-01		2020-01
Repayments of long-term debt		$ 758
Long-term Debt [Member] \| 4.05% Senior Notes due January 2020 [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Stated interest rate										6.05%
Long-term Debt [Member] \| 4.10% Senior Notes due March 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument						$ 1,000		$ 1,000
Unamortized Issuance Expense						(5)		(7)
Total						$ 995		$ 993
Stated interest rate						4.10%		4.10%
Maturity date, year and month						2022-03		2022-03
Long-term Debt [Member] \| 4.95% Senior Notes due January 2025 [Member]
Debt Instrument [Line Items]
Debt Instrument						$ 1,000		$ 1,000
Unamortized Issuance Expense						(8)		(8)
Unamortized Debt Discount						(1)		(2)
Total						$ 991		$ 990
Stated interest rate			6.70%	4.95%		4.95%		4.95%	6.20%
Maturity date, year and month				2025-01		2025-01		2025-01
Long-term Debt [Member] \| 4.95% Senior Notes due January 2025 [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Stated interest rate										6.95%
Long-term Debt [Member] \| 7.50% Senior Notes due April 2026 [Member]
Debt Instrument [Line Items]
Debt Instrument						$ 650		$ 650
Unamortized Issuance Expense						(9)		(10)
Total						$ 641		$ 640
Stated interest rate		7.50%				7.50%	7.50%	7.50%
Maturity date, year and month		2026-04				2026-04		2026-04
Long-term Debt [Member] \| 7.75% Senior Notes due October 2027 [Member]
Debt Instrument [Line Items]
Debt Instrument						$ 500		$ 500
Unamortized Issuance Expense						(7)		(7)
Total						$ 493		$ 493
Stated interest rate		7.75%				7.75%	7.75%	7.75%
Maturity date, year and month		2027-10				2027-10		2027-10
Senior Notes [Member]
Debt Instrument [Line Items]
Increase in basis spread			1.75%
Over LIBOR [Member] \| Senior Notes [Member]
Debt Instrument [Line Items]
Increase in basis spread			1.75%
Scenario, Forecast [Member] \| Long-term Debt [Member] \| 4.05% Senior Notes due January 2020 [Member]
Debt Instrument [Line Items]
Repayments of long-term debt					$ 40
Scenario, Forecast [Member] \| Long-term Debt [Member] \| 4.10% Senior Notes due March 2022 [Member]
Debt Instrument [Line Items]
Repayments of long-term debt					787
Scenario, Forecast [Member] \| Long-term Debt [Member] \| 4.95% Senior Notes due January 2025 [Member]
Debt Instrument [Line Items]
Repayments of long-term debt					73
Scenario, Forecast [Member] \| Senior Notes [Member]
Debt Instrument [Line Items]
Repayments of long-term debt					$ 900