Employee Benefit Plans - Schedule of Change in Benefit Obligations (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Change in benefit obligations
Actuarial gain	$ (8)	$ 38	$ (68)	$ (42)
Amounts recognized in the Consolidated Balance Sheets
Noncurrent benefit liabilities		333	342
Pension benefits
Change in benefit obligations
Benefit obligations at beginning of period	3,138	2,381	3,138
Service cost		2	2	3
Interest cost		75	86	11
Participant contributions		0	0
Benefits paid		(146)	(210)
Actuarial gain		(95)	(60)
Currency exchange rate changes		(3)	4
Transfers in		9	6
Settlements		(136)	(585)
Benefit obligations at end of period		2,087	2,381	3,138
Change in plan assets
Fair value of plan assets at beginning of period	2,307	2,081	2,307
Actual return on plan assets		(30)	148
Employer contributions		16	412
Participant contributions		0	0
Benefits paid		(146)	(210)
Currency exchange rate changes		(3)	3
Settlements		(136)	(585)
Transfers in		10	6
Fair value of plan assets at end of period		1,792	2,081	2,307
Unfunded status of the plans as of September 30		295	300
Amounts recognized in the Consolidated Balance Sheets
Current benefit liabilities		10	11
Noncurrent benefit liabilities		285	289
Net amount recognized		295	300
Amounts recognized in accumulated other comprehensive income (loss)
Prior service cost (credit)		$ 2	$ 2
Weighted-average plan assumptions
Discount rate		4.28%	3.76%
Rate of compensation increase		3.10%	3.13%
Other postretirement benefits
Change in benefit obligations
Benefit obligations at beginning of period	73	$ 57	$ 73
Service cost		0	0	0
Interest cost		2	1	0
Participant contributions		0	3
Benefits paid		(7)	(16)
Actuarial gain		0	(5)
Currency exchange rate changes		(1)	1
Transfers in		0	0
Settlements		0	0
Benefit obligations at end of period		51	57	73
Change in plan assets
Fair value of plan assets at beginning of period	$ 0	0	0
Actual return on plan assets		0	0
Employer contributions		7	13
Participant contributions		0	3
Benefits paid		(7)	(16)
Currency exchange rate changes		0	0
Settlements		0	0
Transfers in		0	0
Fair value of plan assets at end of period		0	0	$ 0
Unfunded status of the plans as of September 30		51	57
Amounts recognized in the Consolidated Balance Sheets
Current benefit liabilities		6	8
Noncurrent benefit liabilities		45	49
Net amount recognized		51	57
Amounts recognized in accumulated other comprehensive income (loss)
Prior service cost (credit)		$ (56)	$ (68)
Weighted-average plan assumptions
Discount rate		4.08%	3.48%
Rate of compensation increase		0.00%	0.00%