Earnings Per Share (Narrative) (Details) - USD ($) $ / shares in Units, $ in Millions			1 Months Ended
	Sep. 01, 2021	Nov. 13, 2020	Sep. 30, 2021	Aug. 31, 2020
Indigo Natural Resources, LLC
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Shares of Southwestern common stock issued	337,827,171		337,827,171
Value of Southwestern's stock issued as consideration			$ 1,588
NYSE closing price per share of Southwestern common shares (in dollars per share)	$ 4.70
Montage Resources Corporation
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
Shares of Southwestern common stock issued		69,740,848
Value of Southwestern's stock issued as consideration		$ 213
NYSE closing price per share of Southwestern common shares (in dollars per share)		$ 3.05
Montage Resources Corporation | Public Stock Offering
Earnings Per Share, Basic, by Common Class, Including Two Class Method [Line Items]
NYSE closing price per share of Southwestern common shares (in dollars per share)				$ 2.50
Underwritten public offering of common stock (in shares)				63,250,000
Net proceeds from public offering				$ 152