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Unaudited Quarterly Financial Information Unaudited Quarterly Financial Information (Details) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Sep. 30, 2018
Jun. 30, 2018
Mar. 31, 2018
Dec. 31, 2017
Sep. 30, 2017
Jun. 30, 2017
Mar. 31, 2017
Dec. 31, 2016
Sep. 30, 2018
Sep. 30, 2017
Sep. 30, 2016
Quarterly Financial Information Disclosure [Abstract]                      
Operating revenues $ 3,185 $ 2,707 $ 2,792 $ 2,549 $ 3,075 $ 2,571 $ 2,547 $ 2,546 $ 11,233 $ 10,739 $ 10,616
Operating expenses 3,001 1,942 2,027 1,951 2,624 2,010 2,013 2,117 8,921 8,764 8,290
Operating income 184 765 765 598 451 561 534 429 2,312 1,975 2,326
Net income (loss) $ (101) $ 470 $ 462 $ 288 $ 139 $ 233 $ 211 $ 102 $ 1,119 $ 685 $ 1,233