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Selected Quarterly Financial Data (Unaudited) - Summary of Selected Quarterly FInancial Data (Unaudited) (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 revenue $ 3,412 $ 2,489 $ 1,475 $ 7,979 $ 10,888 $ 11,380 $ 11,402 $ 9,589 $ 15,355 $ 43,259 $ 41,303
Loss from operations (2,135) (1,615) (1,637) (972) 861 1,473 1,472 495 (6,359) 4,301 3,229
Net income (loss) $ (1,897) $ (1,841) $ (1,627) $ (1,704) $ 641 $ 1,024 $ 1,052 $ 292 $ (7,069) $ 3,009 $ 2,122
Earnings per share, basic (in dollars per share) $ (6.39) $ (6.33) $ (5.79) $ (6.86) $ 2.54 $ 4.01 $ 4.03 $ 1.09 $ (25.30) $ 11.63 $ 7.70
Earnings per share, diluted (in dollars per share)         $ 2.53 $ 3.99 $ 4.02 $ 1.09 $ (25.30) $ 11.58 $ 7.67