Employee Benefit Plans - Schedule of Fair Values of Plan Assets by Investment Category (Details) - Pension benefits - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,045	$ 1,943	$ 1,792
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	102	166
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,421	1,281
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	7	$ 4
Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	510	489
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	102	166
Cash and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	102	166
Cash and cash equivalents | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash and cash equivalents | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Cash and cash equivalents | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. government securities and futures
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	172	162
U.S. government securities and futures | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. government securities and futures | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	172	162
U.S. government securities and futures | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
U.S. government securities and futures | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other government securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	47	41
Other government securities | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other government securities | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	47	41
Other government securities | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other government securities | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate debt instruments
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,201	1,075
Corporate debt instruments | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate debt instruments | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,201	1,075
Corporate debt instruments | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Corporate debt instruments | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	7
Insurance contracts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Insurance contracts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Insurance contracts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	12	7
Insurance contracts | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Private equity and hedge funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	15
Private equity and hedge funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Private equity and hedge funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Private equity and hedge funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Private equity and hedge funds | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	15
Common collective trusts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	497	474
Common collective trusts | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Common collective trusts | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Common collective trusts | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Common collective trusts | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	497	474
Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	3
Other Investments [Member] | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Investments [Member] | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1	3
Other Investments [Member] | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Other Investments [Member] | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 0	$ 0