Debt (Components Of Debt) (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2016	Dec. 31, 2015
Debt Instrument [Line Items]
Debt Instrument	$ 6,469	$ 4,733
Unamortized Issuance Costs	(23)	(24)
Unamortized Debt Discount	(3)	(4)
Total	6,443	4,705
Short-term Debt [Member]
Debt Instrument [Line Items]
Debt Instrument	$ 1	$ 1
Unamortized Issuance Costs
Unamortized Debt Discount
Total	$ 1	$ 1
Short-term Debt [Member] \| 7.15% Senior Notes due May 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument	$ 1	$ 1
Unamortized Issuance Costs
Unamortized Debt Discount
Total	$ 1	$ 1
Stated interest rate	7.15%	7.15%
Maturity date, month and year	2018-05	2018-05
Long-term Debt [Member]
Debt Instrument [Line Items]
Debt Instrument	$ 6,468	$ 4,732
Unamortized Issuance Costs	(23)	(24)
Unamortized Debt Discount	(3)	(4)
Total	6,442	4,704
Long-term Debt [Member] \| Credit Facility [Member]
Debt Instrument [Line Items]
Debt Instrument	$ 1,852	$ 116
Unamortized Issuance Costs
Unamortized Debt Discount
Total	$ 1,852	$ 116
Credit facility, variable interest rate	4.154%	1.886%
Credit facility, maturity date	Dec. 01, 2018	Dec. 01, 2018
Long-term Debt [Member] \| Term Loan due November 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument	$ 750	$ 750
Unamortized Issuance Costs	$ (2)	$ (3)
Unamortized Debt Discount
Total	$ 748	$ 747
Variable interest rate	2.025%	1.775%
Maturity date, month and year	2018-11	2018-11
Long-term Debt [Member] \| 7.35% Senior Notes due October 2017 [Member]
Debt Instrument [Line Items]
Debt Instrument	$ 15	$ 15
Unamortized Issuance Costs
Unamortized Debt Discount
Total	$ 15	$ 15
Stated interest rate	7.35%	7.35%
Maturity date, month and year	2017-10	2017-10
Long-term Debt [Member] \| 7.125% Senior Notes due October 2017 [Member]
Debt Instrument [Line Items]
Debt Instrument	$ 25	$ 25
Unamortized Issuance Costs
Unamortized Debt Discount
Total	$ 25	$ 25
Stated interest rate	7.125%	7.125%
Maturity date, month and year	2017-10	2017-10
Long-term Debt [Member] \| 3.3% Senior Notes due January 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument	$ 350	$ 350
Unamortized Issuance Costs	$ (2)	$ (2)
Unamortized Debt Discount
Total	$ 348	$ 348
Stated interest rate	3.30%	3.30%
Maturity date, month and year	2018-01	2018-01
Long-term Debt [Member] \| 7.5% Senior Notes due February 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument	$ 600	$ 600
Unamortized Issuance Costs	$ (2)	$ (2)
Unamortized Debt Discount
Total	$ 598	$ 598
Stated interest rate	7.50%	7.50%
Maturity date, month and year	2018-02	2018-02
Long-term Debt [Member] \| 7.15% Senior Notes due May 2018 [Member]
Debt Instrument [Line Items]
Debt Instrument	$ 26	$ 26
Unamortized Issuance Costs
Unamortized Debt Discount
Total	$ 26	$ 26
Stated interest rate	7.15%	7.15%
Maturity date, month and year	2018-05	2018-05
Long-term Debt [Member] \| 4.05% Senior Notes due January 2020 [Member]
Debt Instrument [Line Items]
Debt Instrument	$ 850	$ 850
Unamortized Issuance Costs	$ (5)	(5)
Unamortized Debt Discount		(1)
Total	$ 845	$ 844
Stated interest rate	4.05%	4.05%
Maturity date, month and year	2020-01	2020-01
Long-term Debt [Member] \| 4.10% Senior Notes due March 2022 [Member]
Debt Instrument [Line Items]
Debt Instrument	$ 1,000	$ 1,000
Unamortized Issuance Costs	(5)	(5)
Unamortized Debt Discount	(1)	(1)
Total	$ 994	$ 994
Stated interest rate	4.10%	4.10%
Maturity date, month and year	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 Costs	(7)	(7)
Unamortized Debt Discount	(2)	(2)
Total	$ 991	$ 991
Stated interest rate	4.95%	4.95%
Maturity date, month and year	2025-01	2025-01

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

No definition available.

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

- Definition

Amount of long-term debt before deduction of unamortized discount or premium. Includes, but is not limited to, notes payable, bonds payable, commercial loans, mortgage loans, convertible debt, subordinated debt and other types of debt, with initial maturities beyond one year or the normal operating cycle, if longer.

+ 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 SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 20
-Article 5

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 03
-Paragraph 16
-Article 9

+ 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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28551-108399

Reference 2: 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(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

+ 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

The amount of debt discount that was originally recognized at the issuance of the instrument that has yet to be amortized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 45
-Paragraph 1A
-URI http://asc.fasb.org/extlink&oid=6451184&loc=d3e28541-108399

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 835
-SubTopic 30
-Section 55
-Paragraph 8
-URI http://asc.fasb.org/extlink&oid=34725769&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

+ Details

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

- Definition

The effective interest rate at the end of the reporting period.

+ References

+ Details

Name:	us-gaap_LineOfCreditFacilityInterestRateAtPeriodEnd
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Carrying amount of long-term debt, net of unamortized discount or premium, including current and noncurrent amounts. 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/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 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 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

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Regulation S-X (SX)
-Number 210
-Section 02
-Paragraph 20, 22
-Article 5

+ Details

Name:	us-gaap_LongTermDebt
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

+ Details

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

- Details

Name:	us-gaap_ShortTermDebtTypeAxis=us-gaap_ShortTermDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DebtInstrumentAxis=swn_SeniorNotesFiveMember
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_CreditFacilityAxis=us-gaap_RevolvingCreditFacilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_DebtInstrumentAxis=swn_SeniorNotesFourMember
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: