Long-term Incentive Compensation (Schedule of Liability-Classified Stock-Based Compensation Costs) (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Share-based Payment Arrangement, Noncash Expense [Abstract]
Liability-classified stock-based compensation cost – expensed	$ 13	$ (1)
Liability-classified stock-based compensation cost – capitalized	$ 5	$ 0

Represents the compensation cost capitalized during the period arising from liability-based compensation arrangements (for example, shares of stock, units, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

No definition available.

Name:	swn_LiabilityBasedShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount
Namespace Prefix:	swn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration