Acquisition - (Schedule of Consideration Paid to Stockholders of Montage) (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended
	Nov. 13, 2020	Sep. 30, 2021	Dec. 31, 2020
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Common stock, par value (in dollars per share)	$ 0.01	$ 0.01	$ 0.01
Increase in Southwestern equity		$ 1,588
Common Stock
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Increase in Southwestern equity		4
Additional Paid-In Capital
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Increase in Southwestern equity		$ 1,584
Montage Resources Corporation
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Shares of Southwestern common stock issued in respect of outstanding Montage common stock	67,311,166
Shares of Southwestern common stock issued in respect of Montage stock-based awards	2,429,682
Shares of Southwestern common stock issued	69,740,848
NYSE closing price per share of Southwestern common shares on November 13, 2020 (in dollars per share)	$ 3.05
Business acquisition, equity interest issued	$ 213
Montage Resources Corporation | Common Stock
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Increase in Southwestern equity	1
Montage Resources Corporation | Additional Paid-In Capital
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Increase in Southwestern equity	$ 212