Accumulated Other Comprehensive Loss (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2017
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	$ 6,050	$ 2,084	$ 2,084
Other comprehensive income (loss) before reclassifications	14		(25)
Amounts reclassified from accumulated other comprehensive loss	13		17
Net current period other comprehensive income (loss)	27	(8)	(8)
Ending balance	6,361	2,208	6,050
Foreign Exchange Translation Adjustment
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	1	(76)	(76)
Other comprehensive income (loss) before reclassifications	15		(21)
Amounts reclassified from accumulated other comprehensive loss	0		0
Net current period other comprehensive income (loss)	15		(21)
Ending balance	16	(97)	1
Pension and Other Postretirement Adjustments
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(1,575)	(1,818)	(1,818)
Other comprehensive income (loss) before reclassifications	0		0
Amounts reclassified from accumulated other comprehensive loss	14	16	16
Net current period other comprehensive income (loss)	14		16
Ending balance	(1,561)	(1,802)	(1,575)
Reclassifications from AOCL to net income, related to amortization of net actuarial losses and prior service credits for retirement benefit plans, before Tax	22	25
Change in the Fair Value of Effective Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(1)	(4)	(4)
Other comprehensive income (loss) before reclassifications	(1)		(4)
Amounts reclassified from accumulated other comprehensive loss	(1)		1
Net current period other comprehensive income (loss)	(2)		(3)
Ending balance	(3)	(7)	(1)
Total
Accumulated Other Comprehensive Income (Loss), Net of Tax [Roll Forward]
Beginning balance	(1,575)	(1,898)	(1,898)
Net current period other comprehensive income (loss)	27	(8)
Ending balance	$ (1,548)	$ (1,906)	$ (1,575)