Pension and Postretirement Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Interest cost	$ 1	$ 1
Expected return on plan assets	(2)	(2)
Amortization of prior service credit	(1)	(1)
Net periodic benefit credit	(2)	(2)
UNITED STATES | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	11	18
Expected return on plan assets	(42)	(44)
Net periodic benefit credit	(31)	(26)
Foreign Plan [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	7	6
Interest cost	8	9
Expected return on plan assets	(28)	(27)
Net periodic benefit credit	$ (13)	$ (12)