Pension and Postretirement Plans - Components of Net Periodic Benefit Cost (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Net actuarial (gain) loss	$ (203)
Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1	$ 0	$ 1	$ 0
Interest cost	0	1	1	2
Expected return on plan assets	(2)	(2)	(4)	(4)
Amortization of prior service credit	(1)	0	(2)	(1)
Net periodic benefit credit	(2)	(1)	(4)	(3)
UNITED STATES | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Interest cost	11	19	22	37
Expected return on plan assets	(42)	(46)	(84)	(90)
Net actuarial (gain) loss	(203)	0	(203)	0
Settlement (gain) loss	(4)	0	(4)	0
Net periodic benefit credit	(238)	(27)	(269)	(53)
Foreign Plan [Member] | Pension Plan [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	6	7	13	13
Interest cost	8	9	16	18
Expected return on plan assets	(27)	(29)	(55)	(56)
Amortization of prior service credit	0	1	0	1
Net periodic benefit credit	$ (13)	$ (12)	$ (26)	$ (24)