Stockholders' Deficit - Schedule of Reclassifications From Accumulated Other Comprehensive Income (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of pension and other postretirement plan prior service credit	$ 38				$ (60)			$ (8)	$ 0	$ (138)	$ (35)
Loss on liquidation of subsidiary									(33)	(25)	(19)
Income tax expense (benefit)	2	$ 3	$ 2	$ 71					166	186	148
Net of tax	45	$ 64	$ 67	$ (10)	105	$ 56	$ 71	$ 72	166	304	$ 273
Reclassification of income tax effects of U.S. tax reform	(399)				(167)				(399)	(167)
Total amounts reclassified, net of tax									(11)	46
Reclassification out of Accumulated Other Comprehensive Income
Accumulated Other Comprehensive Income (Loss) [Line Items]
Amortization of pension and other postretirement plan prior service credit									(12)	(12)
Loss on liquidation of subsidiary									1	0
Income tax expense (benefit)									2	4
Net of tax									(9)	(8)
Reclassification of income tax effects of U.S. tax reform	$ 8				$ 0				8	0
Total amounts reclassified, net of tax									$ (1)	$ (8)