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Unaudited Quarterly Financial Information Unaudited Quarterly Financial Information (Details) - USD ($)
$ in Millions
 3 Months Ended 12 Months Ended
Sep. 30, 2015
Jun. 30, 2015
Mar. 31, 2015
Dec. 31, 2014
Sep. 30, 2014
Jun. 30, 2014
Mar. 31, 2014
Dec. 31, 2013
Sep. 30, 2015
Sep. 30, 2014
Sep. 30, 2013
Quarterly Financial Information Disclosure [Abstract]                      
Operating revenues $ 3,171 $ 2,558 $ 2,863 $ 2,411 $ 3,166 $ 2,651 $ 2,938 $ 2,382 $ 11,003 $ 11,137 $ 10,956
Operating expenses 2,402 2,252 2,087 2,047 2,569 2,453 2,362 2,164 8,788 9,548 9,503
Operating income 769 306 776 364 597 198 576 218 2,215 1,589 1,453
Net income (loss) $ 502 $ 32 $ 496 $ 81 $ 322 $ (81) $ 295 $ (67) $ 1,111 $ 469 $ 271