Plan Assets by Asset Category (Details) - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Pension Plan | United States
Retirement Plans [Line Items]
Fair value of plan assets	$ 2,459	$ 2,706	$ 2,736
Pension Plan | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	2,344	2,213	2,098
Pension Plan | Due to Broker | United States
Retirement Plans [Line Items]
Fair value of plan assets	(77)	(80)
Postretirement Benefits
Retirement Plans [Line Items]
Fair value of plan assets	172	153	$ 163
Postretirement Benefits | Cash and cash equivalents
Retirement Plans [Line Items]
Fair value of plan assets	5	5
Postretirement Benefits | Large-Cap
Retirement Plans [Line Items]
Fair value of plan assets	24	23
Postretirement Benefits | Small-Cap
Retirement Plans [Line Items]
Fair value of plan assets	8	7
Postretirement Benefits | International-Developed
Retirement Plans [Line Items]
Fair value of plan assets	19	16
Postretirement Benefits | International - Emerging
Retirement Plans [Line Items]
Fair value of plan assets	12	10
Postretirement Benefits | Government
Retirement Plans [Line Items]
Fair value of plan assets	20	19
Postretirement Benefits | Corporate/Other
Retirement Plans [Line Items]
Fair value of plan assets	56	53
Postretirement Benefits | Real Estate
Retirement Plans [Line Items]
Fair value of plan assets	11	8
Postretirement Benefits | Commodities
Retirement Plans [Line Items]
Fair value of plan assets	17	12
Fair Value, Inputs, Level 1 | Pension Plan | United States
Retirement Plans [Line Items]
Fair value of plan assets	1,993	2,251
Fair Value, Inputs, Level 1 | Pension Plan | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	615	665
Fair Value, Inputs, Level 1 | Pension Plan | Cash and cash equivalents | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Pension Plan | Cash and cash equivalents | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	151	178
Fair Value, Inputs, Level 1 | Pension Plan | Large-Cap | United States
Retirement Plans [Line Items]
Fair value of plan assets	185	198
Fair Value, Inputs, Level 1 | Pension Plan | Large-Cap | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	23	23
Fair Value, Inputs, Level 1 | Pension Plan | Small-Cap | United States
Retirement Plans [Line Items]
Fair value of plan assets	215	255
Fair Value, Inputs, Level 1 | Pension Plan | International-Developed | United States
Retirement Plans [Line Items]
Fair value of plan assets	182	220
Fair Value, Inputs, Level 1 | Pension Plan | International-Developed | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	30	52
Fair Value, Inputs, Level 1 | Pension Plan | International - Emerging | United States
Retirement Plans [Line Items]
Fair value of plan assets	34	33
Fair Value, Inputs, Level 1 | Pension Plan | International - Emerging | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Pension Plan | Government | United States
Retirement Plans [Line Items]
Fair value of plan assets	98	159
Fair Value, Inputs, Level 1 | Pension Plan | Government | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	77	64
Fair Value, Inputs, Level 1 | Pension Plan | Corporate/Other | United States
Retirement Plans [Line Items]
Fair value of plan assets	1,279	1,386
Fair Value, Inputs, Level 1 | Pension Plan | Corporate/Other | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	320	321
Fair Value, Inputs, Level 1 | Pension Plan | Real Estate | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	14	27
Fair Value, Inputs, Level 1 | Pension Plan | Hedge Funds | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Postretirement Benefits
Retirement Plans [Line Items]
Fair value of plan assets	5	5
Fair Value, Inputs, Level 1 | Postretirement Benefits | Cash and cash equivalents
Retirement Plans [Line Items]
Fair value of plan assets	5	5
Fair Value, Inputs, Level 1 | Postretirement Benefits | Large-Cap
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Postretirement Benefits | Small-Cap
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Postretirement Benefits | International-Developed
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Postretirement Benefits | International - Emerging
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Postretirement Benefits | Government
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Postretirement Benefits | Corporate/Other
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Postretirement Benefits | Real Estate
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 1 | Postretirement Benefits | Commodities
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) | Pension Plan | United States
Retirement Plans [Line Items]
Fair value of plan assets	263	259
Significant Other Observable Inputs (Level 2) | Pension Plan | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	1,624	1,532
Significant Other Observable Inputs (Level 2) | Pension Plan | Cash and cash equivalents | United States
Retirement Plans [Line Items]
Fair value of plan assets	75	36
Significant Other Observable Inputs (Level 2) | Pension Plan | Cash and cash equivalents | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) | Pension Plan | Large-Cap | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) | Pension Plan | Large-Cap | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	174	334
Significant Other Observable Inputs (Level 2) | Pension Plan | Small-Cap | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) | Pension Plan | International-Developed | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) | Pension Plan | International-Developed | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	98	174
Significant Other Observable Inputs (Level 2) | Pension Plan | International - Emerging | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) | Pension Plan | International - Emerging | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	2	4
Significant Other Observable Inputs (Level 2) | Pension Plan | Government | United States
Retirement Plans [Line Items]
Fair value of plan assets	188	223
Significant Other Observable Inputs (Level 2) | Pension Plan | Government | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	1,046	640
Significant Other Observable Inputs (Level 2) | Pension Plan | Corporate/Other | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) | Pension Plan | Corporate/Other | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	277	331
Significant Other Observable Inputs (Level 2) | Pension Plan | Real Estate | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) | Pension Plan | Hedge Funds | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	27	49
Significant Other Observable Inputs (Level 2) | Postretirement Benefits
Retirement Plans [Line Items]
Fair value of plan assets	167	148
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | Cash and cash equivalents
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | Large-Cap
Retirement Plans [Line Items]
Fair value of plan assets	24	23
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | Small-Cap
Retirement Plans [Line Items]
Fair value of plan assets	8	7
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | International-Developed
Retirement Plans [Line Items]
Fair value of plan assets	19	16
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | International - Emerging
Retirement Plans [Line Items]
Fair value of plan assets	12	10
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | Government
Retirement Plans [Line Items]
Fair value of plan assets	20	19
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | Corporate/Other
Retirement Plans [Line Items]
Fair value of plan assets	56	53
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | Real Estate
Retirement Plans [Line Items]
Fair value of plan assets	11	8
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | Commodities
Retirement Plans [Line Items]
Fair value of plan assets	17	12
Fair Value, Inputs, Level 3 | Pension Plan | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | Cash and cash equivalents | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | Cash and cash equivalents | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | Large-Cap | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | Large-Cap | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | Small-Cap | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | International-Developed | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | International-Developed | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | International - Emerging | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | International - Emerging | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | Government | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | Government | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | Corporate/Other | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | Corporate/Other | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | Real Estate | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Pension Plan | Hedge Funds | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Postretirement Benefits
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Postretirement Benefits | Cash and cash equivalents
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Postretirement Benefits | Large-Cap
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Postretirement Benefits | Small-Cap
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Postretirement Benefits | International-Developed
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Postretirement Benefits | International - Emerging
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Postretirement Benefits | Government
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Postretirement Benefits | Corporate/Other
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Postretirement Benefits | Real Estate
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value, Inputs, Level 3 | Postretirement Benefits | Commodities
Retirement Plans [Line Items]
Fair value of plan assets	0	0
Fair Value Measured at Net Asset Value Per Share [Member] | Pension Plan | Real Estate | United States
Retirement Plans [Line Items]
Fair value of plan assets	280	276
Fair Value Measured at Net Asset Value Per Share [Member] | Pension Plan | Real Estate | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	105	16
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | United States
Retirement Plans [Line Items]
Fair value of plan assets	2,256	2,510
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	2,239	2,197
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Cash and cash equivalents | United States
Retirement Plans [Line Items]
Fair value of plan assets	75	36
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Cash and cash equivalents | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	151	178
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Large-Cap | United States
Retirement Plans [Line Items]
Fair value of plan assets	185	198
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Large-Cap | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	197	357
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Small-Cap | United States
Retirement Plans [Line Items]
Fair value of plan assets	215	255
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | International-Developed | United States
Retirement Plans [Line Items]
Fair value of plan assets	182	220
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | International-Developed | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	128	226
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | International - Emerging | United States
Retirement Plans [Line Items]
Fair value of plan assets	34	33
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | International - Emerging | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	2	4
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Government | United States
Retirement Plans [Line Items]
Fair value of plan assets	286	382
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Government | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	1,123	704
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Corporate/Other | United States
Retirement Plans [Line Items]
Fair value of plan assets	1,279	1,386
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Corporate/Other | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	597	652
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Real Estate | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	14	27
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Hedge Funds | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	$ 27	$ 49