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