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Quarterly Financial Data (Unaudited) - Quarterly Results Of Operations (Details) - USD ($)
$ / shares in Units, $ in Millions
 3 Months Ended 12 Months Ended
Dec. 31, 2020
Sep. 30, 2020
Jun. 30, 2020
Mar. 31, 2020
Dec. 31, 2019
Sep. 30, 2019
Jun. 30, 2019
Mar. 31, 2019
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]                      
Operating revenues $ 661 $ 492 $ 215 $ 1,588 $ 2,031 $ 2,086 $ 2,105 $ 1,871 $ 2,957 $ 8,094 $ 7,658
Operating income (loss) (454) (516) (410) (334) 227 247 250 76 (1,714) 800 266
Net (loss) income $ (373) $ (393) $ (320) $ (268) $ 161 $ 187 $ 179 $ 42 $ (1,354) $ 569 $ 189
Basic (in dollars per share) $ (1.31) $ (1.44) $ (1.18) $ (0.97) $ 0.56 $ 0.63 $ 0.60 $ 0.14 $ (4.88) $ 1.92 $ 0.60
Diluted (in dollars per share)         $ 0.56 $ 0.63 $ 0.59 $ 0.14 $ (4.88) $ 1.91 $ 0.60