Benefit Plans - Fair Value Measurements of U.S. Plan Assets (Detail) - Pension Plans - USD ($) $ in Millions	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2016
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 2,642	$ 1,932	$ 1,855
United States
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,821	1,254
United States | Mortgage and asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	28	155
United States | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	484	232
United States | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	257	107
United States | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	122	98
United States | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	536	369
United States | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	39	40
United States | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	356	252
Quoted Prices in Active Markets for Identical Assets (Level 1) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	600	371
Quoted Prices in Active Markets for Identical Assets (Level 1) | United States | Mortgage and asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Quoted Prices in Active Markets for Identical Assets (Level 1) | United States | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	101	89
Quoted Prices in Active Markets for Identical Assets (Level 1) | United States | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	199	83
Quoted Prices in Active Markets for Identical Assets (Level 1) | United States | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	85	63
Quoted Prices in Active Markets for Identical Assets (Level 1) | United States | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	176	62
Quoted Prices in Active Markets for Identical Assets (Level 1) | United States | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	39	40
Quoted Prices in Active Markets for Identical Assets (Level 1) | United States | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	34
Significant Other Observable Inputs (Level 2) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	497	346
Significant Other Observable Inputs (Level 2) | United States | Mortgage and asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	28	155
Significant Other Observable Inputs (Level 2) | United States | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	383	144
Significant Other Observable Inputs (Level 2) | United States | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	57	25
Significant Other Observable Inputs (Level 2) | United States | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	28	22
Significant Other Observable Inputs (Level 2) | United States | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) | United States | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Other Observable Inputs (Level 2) | United States | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) | United States | Mortgage and asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) | United States | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) | United States | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) | United States | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) | United States | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) | United States | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Significant Unobservable Inputs (Level 3) | United States | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value [Member] | United States
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	724	537
Net Asset Value [Member] | United States | Mortgage and asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value [Member] | United States | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value [Member] | United States | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value [Member] | United States | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	8	12
Net Asset Value [Member] | United States | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	360	307
Net Asset Value [Member] | United States | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	0	0
Net Asset Value [Member] | United States | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 356	$ 217