Acquisitions and Divestitures - (Schedule of the Allocation of Purchase Price) (Details) - Montage Resources Corporation $ in Millions	Nov. 13, 2020 USD ($)
Consideration:
Fair value of Southwestern’s stock issued on November 13, 2020	$ 213
Cash and cash equivalents	3
Accounts receivable	73
Other current assets	1
Derivative assets	11
Evaluated natural gas and oil properties	1,012
Unevaluated natural gas and oil properties	100
Other property, plant and equipment	28
Other long-term assets	26
Total assets acquired	1,254
Accounts payable	155
Other current liabilities	49
Derivative liabilities	70
Revolving credit facility	200
Senior unsecured notes	522
Asset retirement obligations	28
Other long-term liabilities	17
Total liabilities assumed	1,041
Net assets acquired and liabilities assumed	213
Purchase price adjustment	$ 10