Stockholders' Deficit - Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2020	Sep. 30, 2019
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ (258)	$ (358)
Balance at end of period	(76)	(258)
Unamortized benefit plan credits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	34	43
Other comprehensive income (loss) before reclassification	0	0
(Gain) loss reclassified out of accumulated other comprehensive income	(12)	(12)
Tax benefit (expense)	3	3
Balance at end of period	25	34
Currency translation adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(23)	(11)
Other comprehensive income (loss) before reclassification	6	(11)
(Gain) loss reclassified out of accumulated other comprehensive income	1	(1)
Tax benefit (expense)	0	0
Balance at end of period	(16)	(23)
Changes in fair value of cash flow hedges
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	0	0
Other comprehensive income (loss) before reclassification	(1)	0
(Gain) loss reclassified out of accumulated other comprehensive income	0	0
Tax benefit (expense)	0	0
Balance at end of period	(1)	0
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	11	32
Other comprehensive income (loss) before reclassification	5	(11)
(Gain) loss reclassified out of accumulated other comprehensive income	(11)	(13)
Tax benefit (expense)	3	3
Balance at end of period	$ 8	$ 11