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Derivative Financial Instruments - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended		6 Months Ended		12 Months Ended
	Mar. 31, 2021	Mar. 31, 2020	Mar. 31, 2021	Mar. 31, 2020	Sep. 30, 2020
Derivatives, Fair Value [Line Items]
Companys hedged interest rate transactions			3 years
Deferred (losses) gains on derivative financial instruments	$ 3	$ (23)	$ 6	$ (20)
Interest Rate Swap
Derivatives, Fair Value [Line Items]
Outstanding hedge contracts	820		820		$ 820
Deferred gain (loss) on derivative financial instruments			(23)
Deferred (losses) gains on derivative financial instruments					$ (29)
Unrealized pre-tax (losses) gains on derivative financial instruments			8	(25)
Foreign Exchange Contract | Other Income (Expense)
Derivatives, Fair Value [Line Items]
Realized foreign exchange forward contract gain (loss)			(2)	(2)
Unrealized foreign exchange forward contract gain (loss)			0	$ (3)
Sale
Derivatives, Fair Value [Line Items]
Outstanding hedge contracts	243		243
Purchase
Derivatives, Fair Value [Line Items]
Outstanding hedge contracts	$ 149		$ 149

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Nominal or face amount used to calculate payment on derivative.

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Maximum period the entity is hedging its exposure to the variability in future cash flows for forecasted transactions, excluding those forecasted transactions related to the payment of variable interest on existing financial instruments, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount, after tax and reclassification, of gain (loss) from derivative instrument designated and qualifying as cash flow hedge included in assessment of hedge effectiveness.

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