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

Debt Instrument, Incremental Decrease in Basis Spread on Variable Rate Resulting from Upgrades

+ References

No definition available.

+ Details

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

- Definition

Debt Instrument, Incremental Increase in Basis Spread on Variable Rate Resulting from Downgrades

+ References

No definition available.

+ Details

Name:	swn_DebtInstrumentIncrementalIncreaseInBasisSpreadOnVariableRateResultingFromDowngrades
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 format.

+ References

No definition available.

+ Details

Name:	swn_DebtInstrumentMaturityYear
Namespace Prefix:	swn_
Data Type:	xbrli:gYearItemType
Balance Type:	na
Period Type:	duration

- Definition

Debt Instrument, Percentage of Face Amount, Sold to Public

+ References

No definition available.

+ Details

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

- Definition

Purchase Of Note Principal

+ References

No definition available.

+ Details

Name:	swn_PurchaseOfNotePrincipal
Namespace Prefix:	swn_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

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

+ 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(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

Difference between the fair value of payments made and the carrying amount of debt which is extinguished prior to maturity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=49170532&loc=d3e12317-112629

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=66902209&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 470
-SubTopic 50
-Section 40
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=49170532&loc=d3e12355-112629

+ Details

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

- Definition

The cash inflow during the period from additional borrowings in aggregate debt. Includes proceeds from short-term and long-term debt.

+ References

No definition available.

+ Details

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

- Definition

Carrying value as of the balance sheet date of Notes with the highest claim on the assets of the issuer in case of bankruptcy or liquidation (with maturities initially due after one year or beyond the operating cycle if longer), excluding current portion. Senior note holders are paid off in full before any payments are made to junior note holders.

+ 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

+ Details

Name:	us-gaap_SeniorLongTermNotes
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://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.17)
-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_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_DebtInstrumentAxis=swn_SeniorNotesThreeMember
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_SeniorNotesEightMember
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:

Debt (Senior Notes - Narrative) (Details) - USD ($) $ in Millions	1 Months Ended		3 Months Ended
	Jul. 31, 2016	Jan. 31, 2015	Mar. 31, 2017	Dec. 31, 2016
Debt Instrument [Line Items]
Net loss on extinguishment of debt			$ (1)
Unamortized debt expense			26	$ 28
Long-term Debt [Member]
Debt Instrument [Line Items]
Unamortized debt expense			$ 26	$ 28
Senior Notes [Member]
Debt Instrument [Line Items]
Proceeds from issuance of debt		$ 2,200
Purchase of note principal	$ 700
Senior Notes [Member] \| Over LIBOR [Member]
Debt Instrument [Line Items]
Incremental increase in basis points resulting from downgrades			0.25%
Incremental decrease in basis points resulting from upgrades			0.25%
Increase in basis spread	1.75%
3.30% Senior Notes due January 2018 [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Senior notes		$ 350
Stated interest rate	5.05%	3.30%		3.30%
Maturity date, year		2018
Percentage of face amount, sold to public		99.949%
Unamortized debt expense
4.05% Senior Notes due January 2020 [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Senior notes		$ 850
Stated interest rate	5.80%	4.05%	4.05%	4.05%
Maturity date, year		2020
Percentage of face amount, sold to public		99.897%
Unamortized debt expense			$ 4	$ 5
4.95% Senior Notes due January 2025 [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Senior notes		$ 1,000
Stated interest rate	6.70%	4.95%	4.95%	4.95%
Maturity date, year		2025
Percentage of face amount, sold to public		99.782%
Unamortized debt expense			$ 6	$ 7
Maximum [Member] \| 3.30% Senior Notes due January 2018 [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Stated interest rate			5.30%
Maximum [Member] \| 4.05% Senior Notes due January 2020 [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Stated interest rate			6.05%
Maximum [Member] \| 4.95% Senior Notes due January 2025 [Member] \| Long-term Debt [Member]
Debt Instrument [Line Items]
Stated interest rate			6.95%