Acquisition - (Schedule of the Allocation of Purchase Price of Montage) (Details) - Montage Resources Corporation $ in Millions	Nov. 13, 2020 USD ($)
Consideration:
Fair value of Southwestern’s stock issued on November 13, 2020	$ 213
Fair Value of Assets Acquired:
Cash and cash equivalents	3
Accounts receivable	73
Other current assets	1
Commodity derivative assets	11
Evaluated oil and gas properties	1,012
Unevaluated oil and gas properties	90
Other property, plant and equipment	28
Other long-term assets	26
Total assets acquired	1,244
Fair Value of Liabilities Assumed:
Accounts payable	145
Other current liabilities	49
Derivative liabilities	70
Revolving credit facility	200
Senior unsecured notes	522
Asset retirement obligations	28
Other noncurrent liabilities	17
Total liabilities assumed	1,031
Net Assets Acquired and Liabilities Assumed	$ 213