Retirement Plans - Plan Assets by Asset Category (Details) - Pension plan - USD ($) $ in Millions	Sep. 30, 2023	Sep. 30, 2022	Sep. 30, 2021
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	$ 263	$ 283	$ 493
Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	78	72
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	134	148
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	4	7
Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	47	56
Cash
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	6	2
Cash | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	6	2
Cash | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Cash | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Cash | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Domestic
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	1	1
Domestic | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	1	1
Domestic | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Domestic | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Domestic | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
International - Developed
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	17	18
International - Developed | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	17	14
International - Developed | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
International - Developed | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
International - Developed | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	4
International - Emerging
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value		1
International - Emerging | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value		0
International - Emerging | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value		0
International - Emerging | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value		0
International - Emerging | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value		1
Government
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	128	125
Government | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	30	33
Government | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	75	67
Government | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Government | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	23	25
Corporate/Other
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	59	76
Corporate/Other | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	24	22
Corporate/Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	25	43
Corporate/Other | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Corporate/Other | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	10	11
Hedge Fund
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	34	38
Hedge Fund | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Hedge Fund | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	34	38
Hedge Fund | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Hedge Fund | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Real Estate
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	18	22
Real Estate | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Real Estate | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Real Estate | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	4	7
Real Estate | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	$ 14	$ 15