Pension and Other Postretirement Benefits - Pension Plan Assets (Detail) - USD ($) $ in Millions	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2017
U.S. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	$ 1,090	$ 1,090	$ 1,074
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	1,090	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	18	65
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	580	571
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	188	187
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	35	30
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	99	62
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	34
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	133	141
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	250	268
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	18	65
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	99	62
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	133	141
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	803	788
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	580	571
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	188	187
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	35	30
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	34
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	34
Non-U.S. Plans
Defined Benefit Plans and Other Postretirement Benefit Plans Table Text Block [Line Items]
Fair value of plan assets	490	436	$ 433
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	490	436
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	16	6
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	44	0
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	51	33
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	66	68
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	313	329
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	82	74
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	16	6
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	66	68
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	357	329
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	44
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	313	329
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	51	33
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	$ 51	$ 33