Benefit Plans - Fair Value Measurements of Plan Assets (Detail) - Pension Plans - USD ($) $ in Millions	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2019
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 3,222	$ 3,045	$ 2,926
United States
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2,189	2,108
United States | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	679	656
United States | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	184	95
United States | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	46	40
United States | Other fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	141	141
United States | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	686	737
United States | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	168	147
United States | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	286	291
Foreign Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,033	935
Foreign Plans | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	55	34
Foreign Plans | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	13	3
Foreign Plans | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	264	212
Foreign Plans | Other fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	121	102
Foreign Plans | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	297	335
Foreign Plans | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	14	12
Foreign Plans | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	107	72
Foreign Plans | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	44	34
Foreign Plans | Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	118	131
Net Asset Value | United States
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	195	193
Net Asset Value | United States | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value | United States | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value | United States | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value | United States | Other fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value | United States | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	43	43
Net Asset Value | United States | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value | United States | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	152	151
Net Asset Value | Foreign Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value | Foreign Plans | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value | Foreign Plans | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value | Foreign Plans | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value | Foreign Plans | Other fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value | Foreign Plans | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value | Foreign Plans | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value | Foreign Plans | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value | Foreign Plans | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value | Foreign Plans | Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 1 | United States
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,405	1,330
Level 1 | United States | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	297	273
Level 1 | United States | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	162	74
Level 1 | United States | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 1 | United States | Other fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 1 | United States | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	643	694
Level 1 | United States | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	168	147
Level 1 | United States | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	135	141
Level 1 | Foreign Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	765	611
Level 1 | Foreign Plans | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	37	15
Level 1 | Foreign Plans | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	10	0
Level 1 | Foreign Plans | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	249	115
Level 1 | Foreign Plans | Other fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	72	63
Level 1 | Foreign Plans | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	297	335
Level 1 | Foreign Plans | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	14	12
Level 1 | Foreign Plans | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	84	70
Level 1 | Foreign Plans | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	2	0
Level 1 | Foreign Plans | Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | United States
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	590	584
Level 2 | United States | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	382	383
Level 2 | United States | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22	21
Level 2 | United States | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	46	40
Level 2 | United States | Other fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	141	141
Level 2 | United States | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | United States | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | United States | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Foreign Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	124	183
Level 2 | Foreign Plans | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	18	19
Level 2 | Foreign Plans | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	3	3
Level 2 | Foreign Plans | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15	97
Level 2 | Foreign Plans | Other fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	49	39
Level 2 | Foreign Plans | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Foreign Plans | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 2 | Foreign Plans | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8	1
Level 2 | Foreign Plans | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	31	24
Level 2 | Foreign Plans | Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | United States
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | United States | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | United States | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | United States | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | United States | Other fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | United States | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | United States | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | United States | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Foreign Plans
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	145	141
Level 3 | Foreign Plans | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Foreign Plans | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Foreign Plans | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Foreign Plans | Other fixed income
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Foreign Plans | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Foreign Plans | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Level 3 | Foreign Plans | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	15	0
Level 3 | Foreign Plans | Real estate
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	11	10
Level 3 | Foreign Plans | Insurance contracts
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 118	$ 131