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Leases (Narrative) (Details) $ in Millions	Sep. 30, 2019 USD ($)
Leases [Abstract]
Operating leases not yet commenced (less than)	$ 20

X

- Definition

Lessee, Operating Lease, Not Yet Commenced

+ References

No definition available.

+ Details

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

X

- References

No definition available.

+ Details

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