Benefit Plans - Fair Value Measurements of U.S. Plan Assets (Detail) - Pension Plans - USD ($) $ in Millions	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2015
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	$ 1,932	$ 1,855	$ 1,732
United States
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	1,254	1,231
United States | Mortgage and asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	155	169
United States | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	232	197
United States | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	107	103
United States | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	98	90
United States | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	369	348
United States | Cash and cash equivalents
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	40	89
United States | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	252	234
Quoted Prices in Active Markets for Identical Assets (Level 1) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	371	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	89	68
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	83	67
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	63	52
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	62	61
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	40	89
Quoted Prices in Active Markets for Identical Assets (Level 1) | United States | Other
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	34	33
Significant Other Observable Inputs (Level 2) | United States
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	346	340
Significant Other Observable Inputs (Level 2) | United States | Mortgage and asset-backed securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	155	169
Significant Other Observable Inputs (Level 2) | United States | Corporate bonds
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	144	129
Significant Other Observable Inputs (Level 2) | United States | Government and agency-U.S.
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	25	36
Significant Other Observable Inputs (Level 2) | United States | Government and agency-Foreign
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	22	6
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	537	520
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	12	32
Net Asset Value [Member] | United States | Equity securities
Defined Benefit Plan Disclosure [Line Items]
Fair value of plan assets	307	287
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	$ 217	$ 201