Supplemental Oil and Gas Disclosures (Cost Incurred in Oil and Gas Property Acquisition, Exploration, and Development Activities Disclosure) (Details) $ in Millions	12 Months Ended
Dec. 31, 2019 USD ($) $ / Mcfe	Dec. 31, 2018 USD ($) $ / Mcfe	Dec. 31, 2017 USD ($) $ / Mcfe
Oil and Gas Exploration and Production Industries Disclosures [Abstract]
Unproved property acquisition costs	$ 162	$ 164	$ 194
Exploration costs	2	5	22
Development costs	936	1,014	1,024
Capitalized costs incurred	$ 1,100	$ 1,183	$ 1,240
Full cost pool amortization per Mcfe \| $ / Mcfe	0.56	0.51	0.45

Amount of Full cost pool amortization per Mcfe.

No definition available.

Name:	swn_FullCostPoolAmortizationPerMcfe
Namespace Prefix:	swn_
Data Type:	srt-types:perUnitItemType
Balance Type:	na
Period Type:	duration

- Definition

Cost incurred, including capitalized costs and costs charged to expense, in acquisition of oil and gas properties.

+ References

+ Details

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

- Definition

Costs incurred, including capitalized costs and costs charged to expense, associated with the acquisition of oil and gas properties that have unproved reserves.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 18
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62014-109447

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 932
-SubTopic 235
-Section 50
-Paragraph 19
-URI http://asc.fasb.org/extlink&oid=68064819&loc=d3e62038-109447

+ Details

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

- Definition

Development costs incurred, including capitalized costs and costs charged to expense, in oil and gas activities.

+ References

+ Details

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

- Definition

Exploration costs incurred, including capitalized costs and costs charged to expense, in oil and gas activities.

+ References

+ Details

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

- References

No definition available.

+ Details

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