Commitments, contingencies and guarantees - Maturities of commitments (Detail) - JPY (¥) ¥ in Millions	Mar. 31, 2026	Mar. 31, 2025
Commitments and Contingencies Disclosure [Line Items]
Commitments to extend credit	¥ 2,665,196	¥ 3,238,123
Commitments to invest	66,952	25,677
Liquidity facilities to central clearing counterparties [Member]
Commitments and Contingencies Disclosure [Line Items]
Commitments to extend credit	1,034,593	2,038,836
Other commitments to extend credit [Member]
Commitments and Contingencies Disclosure [Line Items]
Commitments to extend credit	1,630,603	¥ 1,199,287
Less than 1 year [Member]
Commitments and Contingencies Disclosure [Line Items]
Commitments to extend credit	1,412,624
Commitments to invest	2,991
Less than 1 year [Member] \| Liquidity facilities to central clearing counterparties [Member]
Commitments and Contingencies Disclosure [Line Items]
Commitments to extend credit	1,034,593
Less than 1 year [Member] \| Other commitments to extend credit [Member]
Commitments and Contingencies Disclosure [Line Items]
Commitments to extend credit	378,031
1 to 3 years [Member]
Commitments and Contingencies Disclosure [Line Items]
Commitments to extend credit	591,051
Commitments to invest	382
1 to 3 years [Member] \| Liquidity facilities to central clearing counterparties [Member]
Commitments and Contingencies Disclosure [Line Items]
Commitments to extend credit
1 to 3 years [Member] \| Other commitments to extend credit [Member]
Commitments and Contingencies Disclosure [Line Items]
Commitments to extend credit	591,051
3 to 5 years [Member]
Commitments and Contingencies Disclosure [Line Items]
Commitments to extend credit	395,454
Commitments to invest	4,554
3 to 5 years [Member] \| Liquidity facilities to central clearing counterparties [Member]
Commitments and Contingencies Disclosure [Line Items]
Commitments to extend credit
3 to 5 years [Member] \| Other commitments to extend credit [Member]
Commitments and Contingencies Disclosure [Line Items]
Commitments to extend credit	395,454
More than 5 years [Member]
Commitments and Contingencies Disclosure [Line Items]
Commitments to extend credit	266,067
Commitments to invest	59,025
More than 5 years [Member] \| Liquidity facilities to central clearing counterparties [Member]
Commitments and Contingencies Disclosure [Line Items]
Commitments to extend credit
More than 5 years [Member] \| Other commitments to extend credit [Member]
Commitments and Contingencies Disclosure [Line Items]
Commitments to extend credit	¥ 266,067