Derivative Instruments and Hedging Activities - Net Investment Hedges (Detail) - Foreign Currency Denominated Debt [Member] € in Millions, ¥ in Billions	6 Months Ended		12 Months Ended
	Mar. 31, 2021 EUR (€)	Mar. 31, 2021 JPY (¥)	Sep. 30, 2020 EUR (€)	Sep. 30, 2020 JPY (¥)
888 million euro net investment hedge [Member]
Derivative [Line Items]
Notional amount	€ 888		€ 888
500 million euro net investment hedge [Member]
Derivative [Line Items]
Notional amount	500		500
500 million euro net investment hedge #2 [Member]
Derivative [Line Items]
Notional amount	500		500
423 million euro net investment hedge [Member]
Derivative [Line Items]
Notional amount	423		423
54 million euro net investment hedge [Member]
Derivative [Line Items]
Notional amount	€ 54		€ 54
25 billion yen net investment hedge [Member]
Derivative [Line Items]
Notional amount | ¥		¥ 25		¥ 25