ACQUISITION - (Schedule of consideration Paid to Stockholders of Montage) (Details) - USD ($) $ / shares in Units, $ in Millions	Nov. 13, 2020	Mar. 31, 2021	Dec. 31, 2020
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Common stock, par value (in dollars per shares)	$ 0.01	$ 0.01	$ 0.01
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 during Merger	69,740,848
NYSE closing price per share of Southwestern common shares on November 13, 2020 (in dollars per share)	$ 3.05
Total consideration (fair value of Southwestern common shares issued)	$ 213
Montage Resources Corporation | Additional Paid-In Capital
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Montage merger exchange	212
Montage Resources Corporation | Common Stock
Business Acquisition, Equity Interests Issued or Issuable [Line Items]
Montage merger exchange	$ 1