Pension and Other Postretirement Benefits - Pension Plan Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2018
U.S. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	$ 1,142	$ 1,090	$ 1,090
U.S. Plans | Fair Value, Measurements, Recurring
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	1,142	1,090
U.S. Plans | Fair Value, Measurements, Recurring | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	17	18
U.S. Plans | Fair Value, Measurements, Recurring | Collective investment funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	509	580
U.S. Plans | Fair Value, Measurements, Recurring | Corporate debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	373	188
U.S. Plans | Fair Value, Measurements, Recurring | U.S. government-sponsored debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	30	35
U.S. Plans | Fair Value, Measurements, Recurring | U.S. Treasury securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	84	99
U.S. Plans | Fair Value, Measurements, Recurring | Asset-backed securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	37	37
U.S. Plans | Fair Value, Measurements, Recurring | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	92	133
U.S. Plans | Fair Value, Measurements, Recurring | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	193	250
U.S. Plans | Fair Value, Measurements, Recurring | Level 1 | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	17	18
U.S. Plans | Fair Value, Measurements, Recurring | Level 1 | Collective investment funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
U.S. Plans | Fair Value, Measurements, Recurring | Level 1 | Corporate debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
U.S. Plans | Fair Value, Measurements, Recurring | Level 1 | U.S. government-sponsored debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
U.S. Plans | Fair Value, Measurements, Recurring | Level 1 | U.S. Treasury securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	84	99
U.S. Plans | Fair Value, Measurements, Recurring | Level 1 | Asset-backed securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
U.S. Plans | Fair Value, Measurements, Recurring | Level 1 | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	92	133
U.S. Plans | Fair Value, Measurements, Recurring | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	912	803
U.S. Plans | Fair Value, Measurements, Recurring | Level 2 | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
U.S. Plans | Fair Value, Measurements, Recurring | Level 2 | Collective investment funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	509	580
U.S. Plans | Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	373	188
U.S. Plans | Fair Value, Measurements, Recurring | Level 2 | U.S. government-sponsored debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	30	35
U.S. Plans | Fair Value, Measurements, Recurring | Level 2 | U.S. Treasury securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
U.S. Plans | Fair Value, Measurements, Recurring | Level 2 | Asset-backed securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
U.S. Plans | Fair Value, Measurements, Recurring | Level 2 | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
U.S. Plans | Fair Value, Measurements, Recurring | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	37	37
U.S. Plans | Fair Value, Measurements, Recurring | Level 3 | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
U.S. Plans | Fair Value, Measurements, Recurring | Level 3 | Collective investment funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
U.S. Plans | Fair Value, Measurements, Recurring | Level 3 | Corporate debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
U.S. Plans | Fair Value, Measurements, Recurring | Level 3 | U.S. government-sponsored debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
U.S. Plans | Fair Value, Measurements, Recurring | Level 3 | U.S. Treasury securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
U.S. Plans | Fair Value, Measurements, Recurring | Level 3 | Asset-backed securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	37	37
U.S. Plans | Fair Value, Measurements, Recurring | Level 3 | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
Non-U.S. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	525	490	$ 436
Non-U.S. Plans | Fair Value, Measurements, Recurring
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	525	490
Non-U.S. Plans | Fair Value, Measurements, Recurring | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	6	16
Non-U.S. Plans | Fair Value, Measurements, Recurring | Corporate debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	48	44
Non-U.S. Plans | Fair Value, Measurements, Recurring | Asset-backed securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	67	51
Non-U.S. Plans | Fair Value, Measurements, Recurring | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	66
Non-U.S. Plans | Fair Value, Measurements, Recurring | Equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	65	0
Non-U.S. Plans | Fair Value, Measurements, Recurring | Multi-asset securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	339	313
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 1
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	6	82
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 1 | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	6	16
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 1 | Corporate debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 1 | Asset-backed securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 1 | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	66
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 1 | Equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 1 | Multi-asset securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 2
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	452	357
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 2 | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 2 | Corporate debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	48	44
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 2 | Asset-backed securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 2 | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 2 | Equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	65	0
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 2 | Multi-asset securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	339	313
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 3
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	67	51
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 3 | Cash and cash equivalents
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 3 | Corporate debt securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 3 | Asset-backed securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	67	51
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 3 | Equity securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 3 | Equity funds
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	0	0
Non-U.S. Plans | Fair Value, Measurements, Recurring | Level 3 | Multi-asset securities
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	$ 0	$ 0