Derivative instruments and hedging activities - Schedule of information about relevant transactions outstanding (Parenthetical) (Detail) - JPY (¥) ¥ in Millions		Sep. 30, 2025	Mar. 31, 2025
Transfer of Financial Assets Accounted for as Sales [Line Items]
Gross derivative liabilities arising from the transactions at reporting dates	[1]	¥ 20,620,000	¥ 19,342,000
Interest rate contracts [Member]
Transfer of Financial Assets Accounted for as Sales [Line Items]
Gross derivative liabilities arising from the transactions at reporting dates		¥ 21,512	¥ 19,401

[1] Includes the amount of embedded derivatives bifurcated in accordance with ASC 815.