Shareholders' Equity - Changes in Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Accumulated Other Comprehensive Income (Loss) [Line Items]
Employee Benefit Related Adjustment	$ 23	$ 14	$ 37	$ 24
Tax benefit	(22)	(73)	(29)	(86)
NET INCOME	66	242	99	283
Amount Reclassified from Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
NET INCOME	2	2	4	5
Amount Reclassified from Accumulated Other Comprehensive Income | Prior service benefit
Accumulated Other Comprehensive Income (Loss) [Line Items]
Employee Benefit Related Adjustment	(9)	(9)	(18)	(18)
Amount Reclassified from Accumulated Other Comprehensive Income | Actuarial losses
Accumulated Other Comprehensive Income (Loss) [Line Items]
Employee Benefit Related Adjustment	11	11	22	23
Amount Reclassified from Accumulated Other Comprehensive Income | Employee Benefit Related Adjustments
Accumulated Other Comprehensive Income (Loss) [Line Items]
Total before tax	2	2	4	5
Tax benefit	$ 0	$ 0	$ 0	$ 0