Stockholders' Deficit - Other Comprehensive Income (Loss) (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	$ (358)	$ (117)	$ (330)
Balance at end of period	(258)	(358)	(117)
Unamortized benefit plan credits
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	43	44	52
Fiscal year activity	(9)	(1)	(8)
Balance at end of period	34	43	44
Currency translation adjustments
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	(11)	(1)	(55)
Fiscal year activity	(12)	(10)	54
Balance at end of period	(23)	(11)	(1)
Total
AOCI Attributable to Parent, Net of Tax [Roll Forward]
Balance at beginning of period	32	43	(3)
Fiscal year activity	(21)	(11)	46
Balance at end of period	$ 11	$ 32	$ 43