Plan Assets by Asset Category (Details) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
Pension Plan | United States
Retirement Plans [Line Items]
Fair value of plan assets	$ 2,706	$ 2,736	$ 3,046
Pension Plan | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	2,213	2,098	2,117
Pension Plan | Due to Broker | United States
Retirement Plans [Line Items]
Fair value of plan assets	(80)
Postretirement Benefits
Retirement Plans [Line Items]
Fair value of plan assets	153	163	$ 174
Postretirement Benefits | Cash and cash equivalents
Retirement Plans [Line Items]
Fair value of plan assets	5	6
Postretirement Benefits | Large-Cap
Retirement Plans [Line Items]
Fair value of plan assets	23	22
Postretirement Benefits | Small-Cap
Retirement Plans [Line Items]
Fair value of plan assets	7	8
Postretirement Benefits | International-Developed
Retirement Plans [Line Items]
Fair value of plan assets	16	19
Postretirement Benefits | International - Emerging
Retirement Plans [Line Items]
Fair value of plan assets	10	9
Postretirement Benefits | Government
Retirement Plans [Line Items]
Fair value of plan assets	19	20
Postretirement Benefits | Corporate/Other
Retirement Plans [Line Items]
Fair value of plan assets	53	55
Postretirement Benefits | Real Estate
Retirement Plans [Line Items]
Fair value of plan assets	8	11
Postretirement Benefits | Commodities
Retirement Plans [Line Items]
Fair value of plan assets	12	13
Fair Value, Inputs, Level 1 | Pension Plan | United States
Retirement Plans [Line Items]
Fair value of plan assets	2,251	2,175
Fair Value, Inputs, Level 1 | Pension Plan | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	665	641
Fair Value, Inputs, Level 1 | Pension Plan | Cash and cash equivalents | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	24
Fair Value, Inputs, Level 1 | Pension Plan | Cash and cash equivalents | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	178	174
Fair Value, Inputs, Level 1 | Pension Plan | Large-Cap | United States
Retirement Plans [Line Items]
Fair value of plan assets	198	276
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	255	232
Fair Value, Inputs, Level 1 | Pension Plan | International-Developed | United States
Retirement Plans [Line Items]
Fair value of plan assets	220	233
Fair Value, Inputs, Level 1 | Pension Plan | International-Developed | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	52	54
Fair Value, Inputs, Level 1 | Pension Plan | International - Emerging | United States
Retirement Plans [Line Items]
Fair value of plan assets	33	42
Fair Value, Inputs, Level 1 | Pension Plan | International - Emerging | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	0	1
Fair Value, Inputs, Level 1 | Pension Plan | Government | United States
Retirement Plans [Line Items]
Fair value of plan assets	159	47
Fair Value, Inputs, Level 1 | Pension Plan | Government | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	64	69
Fair Value, Inputs, Level 1 | Pension Plan | Corporate/Other | United States
Retirement Plans [Line Items]
Fair value of plan assets	1,386	1,266
Fair Value, Inputs, Level 1 | Pension Plan | Corporate/Other | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	321	289
Fair Value, Inputs, Level 1 | Pension Plan | Real Estate | United States
Retirement Plans [Line Items]
Fair value of plan assets		55
Fair Value, Inputs, Level 1 | Pension Plan | Real Estate | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	27	31
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	6
Fair Value, Inputs, Level 1 | Postretirement Benefits | Cash and cash equivalents
Retirement Plans [Line Items]
Fair value of plan assets	5	6
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	259	359
Significant Other Observable Inputs (Level 2) | Pension Plan | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	1,532	1,443
Significant Other Observable Inputs (Level 2) | Pension Plan | Cash and cash equivalents | United States
Retirement Plans [Line Items]
Fair value of plan assets	36	31
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	334	191
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	33
Significant Other Observable Inputs (Level 2) | Pension Plan | International-Developed | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	174	235
Significant Other Observable Inputs (Level 2) | Pension Plan | International - Emerging | United States
Retirement Plans [Line Items]
Fair value of plan assets	0	10
Significant Other Observable Inputs (Level 2) | Pension Plan | International - Emerging | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	4	11
Significant Other Observable Inputs (Level 2) | Pension Plan | Government | United States
Retirement Plans [Line Items]
Fair value of plan assets	223	285
Significant Other Observable Inputs (Level 2) | Pension Plan | Government | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	640	709
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	331	228
Significant Other Observable Inputs (Level 2) | Pension Plan | Real Estate | United States
Retirement Plans [Line Items]
Fair value of plan assets		0
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	49	69
Significant Other Observable Inputs (Level 2) | Postretirement Benefits
Retirement Plans [Line Items]
Fair value of plan assets	148	157
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	23	22
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | Small-Cap
Retirement Plans [Line Items]
Fair value of plan assets	7	8
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | International-Developed
Retirement Plans [Line Items]
Fair value of plan assets	16	19
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | International - Emerging
Retirement Plans [Line Items]
Fair value of plan assets	10	9
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | Government
Retirement Plans [Line Items]
Fair value of plan assets	19	20
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | Corporate/Other
Retirement Plans [Line Items]
Fair value of plan assets	53	55
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | Real Estate
Retirement Plans [Line Items]
Fair value of plan assets	8	11
Significant Other Observable Inputs (Level 2) | Postretirement Benefits | Commodities
Retirement Plans [Line Items]
Fair value of plan assets	12	13
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 | United States
Retirement Plans [Line Items]
Fair value of plan assets		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	276	202
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	16	14
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | United States
Retirement Plans [Line Items]
Fair value of plan assets	2,510	2,534
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	2,197	2,084
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	36	55
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	178	174
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Large-Cap | United States
Retirement Plans [Line Items]
Fair value of plan assets	198	276
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Large-Cap | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	357	214
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Small-Cap | United States
Retirement Plans [Line Items]
Fair value of plan assets	255	232
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | International-Developed | United States
Retirement Plans [Line Items]
Fair value of plan assets	220	266
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | International-Developed | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	226	289
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | International - Emerging | United States
Retirement Plans [Line Items]
Fair value of plan assets	33	52
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | International - Emerging | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	4	12
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Government | United States
Retirement Plans [Line Items]
Fair value of plan assets	382	332
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Government | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	704	778
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Corporate/Other | United States
Retirement Plans [Line Items]
Fair value of plan assets	1,386	1,266
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Corporate/Other | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	652	517
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Real Estate | United States
Retirement Plans [Line Items]
Fair value of plan assets		55
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Real Estate | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	27	31
Fair Value, Inputs, Level 1, 2 and 3 [Member] | Pension Plan | Hedge Funds | Foreign Plan
Retirement Plans [Line Items]
Fair value of plan assets	$ 49	$ 69