Retirement Plans - Plan Assets by Asset Category (Details) - Pension Plan - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	$ 470	$ 449	$ 491
Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	201	204
Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	202	146
Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	6	6
Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	61	93
Cash
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	31	31
Cash | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	31	31
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
Equity Securities, Domestic
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	15	17
Equity Securities, Domestic | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	2	3
Equity Securities, Domestic | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	7	0
Equity Securities, Domestic | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Equity Securities, Domestic | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	6	14
Equity Securities, International - Developed
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	49	51
Equity Securities, International - Developed | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	36	36
Equity Securities, International - Developed | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	10	0
Equity Securities, International - Developed | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Equity Securities, International - Developed | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	3	15
Equity Securities, International - Emerging
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	5	5
Equity Securities, International - Emerging | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	2	2
Equity Securities, International - Emerging | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	3	0
Equity Securities, International - Emerging | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Equity Securities, International - Emerging | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	3
Fixed Income Securities, Government
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	218	185
Fixed Income Securities, Government | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	75	83
Fixed Income Securities, Government | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	117	67
Fixed Income Securities, Government | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Fixed Income Securities, Government | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	26	35
Fixed Income Securities, Corporate/Other
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	66	61
Fixed Income Securities, Corporate/Other | Quoted Prices in Active Markets (Level 1)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	55	49
Fixed Income Securities, Corporate/Other | Significant Other Observable Inputs (Level 2)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	2
Fixed Income Securities, Corporate/Other | Significant Unobservable Inputs (Level 3)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	0	0
Fixed Income Securities, Corporate/Other | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	11	10
Hedge Fund
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	65	77
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	65	77
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	21	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	6	6
Real Estate | Net Asset Value (NAV)
Defined Benefit Plan Disclosure [Line Items]
Plan assets, fair value	$ 15	$ 16