Employee Benefit Plans - Additional Information (Detail) - USD ($)	3 Months Ended				6 Months Ended
	Mar. 31, 2021	Dec. 31, 2020	Sep. 30, 2020	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020
Defined Benefit Plan Disclosure [Line Items]
Other postretirement benefit costs (gains)					$ 1,000,000	$ 1,000,000
U.S. Defined Benefit Plans [Member]
Defined Benefit Plan Disclosure [Line Items]
Settlement charge		$ 6,000,000	$ 3,000,000
Defined Benefit Plan, Net Periodic Benefit (Cost) Credit, Settlement Gain (Loss), Statement of Income or Comprehensive Income [Extensible List]		us-gaap:OtherNonoperatingIncomeExpenseMember	us-gaap:OtherNonoperatingIncomeExpenseMember
Maximum [Member]
Defined Benefit Plan Disclosure [Line Items]
Other postretirement benefit costs (gains)	$ 1,000,000			$ 1,000,000