Collateralized transactions - Assets Subject to Lien (Detail) - JPY (¥) ¥ in Millions	Sep. 30, 2025	Mar. 31, 2025
Collateralized Transactions Assets Subject to Lien [Line Items]
Assets subject to lien, amount	¥ 2,611,647	¥ 2,515,096
Loans and receivables [Member]
Collateralized Transactions Assets Subject to Lien [Line Items]
Assets subject to lien, amount	687,103	627,090
Trading assets and private equity and debt investments [Member]
Collateralized Transactions Assets Subject to Lien [Line Items]
Assets subject to lien, amount	1,750,966	1,766,083
Office buildings, land, equipment and facilities [Member]
Collateralized Transactions Assets Subject to Lien [Line Items]
Assets subject to lien, amount	6,942	2,933
Non-trading debt securities [Member]
Collateralized Transactions Assets Subject to Lien [Line Items]
Assets subject to lien, amount	157,825	117,655
Investments in and advances to affiliated companies [Member]
Collateralized Transactions Assets Subject to Lien [Line Items]
Assets subject to lien, amount	3	2
Other [Member]
Collateralized Transactions Assets Subject to Lien [Line Items]
Assets subject to lien, amount	¥ 8,808	¥ 1,333