Employee Benefit Plans - Pension and Other Postretirement Benefit Costs (Details) - USD ($) $ in Millions	12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Pension benefits
Net periodic benefit costs (income)
Service cost	$ 2	$ 2	$ 2
Interest cost	81	75	86
Expected return on plan assets	(80)	(103)	(145)
Amortization of prior service credit	0	0	0
Actuarial loss (gain)	61	38	(63)
Net periodic benefit (income) costs	$ 64	$ 12	$ (120)
Weighted-average plan assumptions
Discount rate for service cost	2.92%	2.94%	2.15%
Discount rate for interest cost	4.00%	3.23%	2.84%
Rate of compensation increase	3.06%	3.05%	2.99%
Expected long-term rate of return on plan assets	4.66%	5.17%	6.56%
Pension benefits | United States | Geographic Concentration Risk | Net Periodic Benefit Obligation
Weighted-average plan assumptions
Concentration risk	97.00%
Other postretirement benefits
Net periodic benefit costs (income)
Service cost	$ 0	$ 0	$ 0
Interest cost	2	2	1
Expected return on plan assets	0	0	0
Amortization of prior service credit	(12)	(12)	(12)
Actuarial loss (gain)	8	0	(5)
Net periodic benefit (income) costs	$ (2)	$ (10)	$ (16)
Weighted-average plan assumptions
Discount rate for service cost	3.98%	4.05%	2.95%
Discount rate for interest cost	3.83%	3.11%	2.64%
Rate of compensation increase	0.00%	0.00%	0.00%
Expected long-term rate of return on plan assets	0.00%	0.00%	0.00%
Other postretirement benefits | United States | Geographic Concentration Risk | Net Periodic Benefit Obligation
Weighted-average plan assumptions
Concentration risk	77.00%