Stockholders' Deficit - Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2021	Sep. 30, 2020
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ (76)	$ (258)
Balance at end of period	135	(76)
Unamortized benefit plan credits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	25	34
Other comprehensive income (loss) before reclassification	0	0
Gains reclassified out of accumulated other comprehensive income	(12)	(12)
Tax benefit (expense)	3	3
Balance at end of period	16	25
Currency translation adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(16)	(23)
Other comprehensive income (loss) before reclassification	7	6
Gains reclassified out of accumulated other comprehensive income	0	1
Tax benefit (expense)	0	0
Balance at end of period	(9)	(16)
Changes in fair value of cash flow hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(1)	0
Other comprehensive income (loss) before reclassification	3	(1)
Gains reclassified out of accumulated other comprehensive income	(1)	0
Tax benefit (expense)	0	0
Balance at end of period	1	(1)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	8	11
Other comprehensive income (loss) before reclassification	10	5
Gains reclassified out of accumulated other comprehensive income	(13)	(11)
Tax benefit (expense)	3	3
Balance at end of period	$ 8	$ 8