Retirement Plans - Plan Assets by Asset Category (Details) - Pension plan - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	$ 493	$ 486	$ 470
Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	113	172
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	305	235
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	7	6
Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	68	73
Cash
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	5	37
Cash | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	5	37
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	12	15
Domestic | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	2	1
Domestic | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	2	7
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	8	7
International - Developed
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	44	51
International - Developed | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	27	29
International - Developed | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	7	11
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	10	11
International - Emerging
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	2	3
International - Emerging | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
International - Emerging | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	2	3
International - Emerging | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
International - Emerging | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Government
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	237	219
Government | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	45	63
Government | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	168	128
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	24	28
Corporate/Other
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	82	64
Corporate/Other | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	34	42
Corporate/Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	38	11
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	88	75
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	88	75
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	23	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	7	6
Real Estate | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	$ 16	$ 16