Employee Benefit Plans - Schedule of Fair Values of Plan Assets by Investment Category (Details) - Pension benefits - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,055	$ 2,045	$ 1,943
Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	136	102
Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,534	1,421
Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	58	12	$ 7
Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	327	510
Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	136	102
Cash and cash equivalents | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	136	102
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	96	172
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	96	172
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	58	47
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	58	47
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	1,371	1,201
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	1,371	1,201
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	58	12
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	58	12
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	11	13
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	11	13
Collective trust funds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	316	497
Collective trust funds | Level 1
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Collective trust funds | Level 2
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Collective trust funds | Level 3
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Collective trust funds | Assets measured at NAV
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	316	497
Other Investments [Member]
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	9	1
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	9	1
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