Collateralized transactions - Assets Subject to Lien (Detail) - JPY (¥) ¥ in Millions	Mar. 31, 2017	Mar. 31, 2016
Collateralized Transactions Assets Subject To Lien [Line Items]
Assets subject to lien, amount	¥ 1,820,663	¥ 1,952,439
Loans and receivables [Member]
Collateralized Transactions Assets Subject To Lien [Line Items]
Assets subject to lien, amount	4,268	249
Trading assets [Member]
Collateralized Transactions Assets Subject To Lien [Line Items]
Assets subject to lien, amount	1,580,765	1,755,260
Office buildings, land, equipment and facilities [Member]
Collateralized Transactions Assets Subject To Lien [Line Items]
Assets subject to lien, amount	12,635	5,355
Non-trading debt securities [Member]
Collateralized Transactions Assets Subject To Lien [Line Items]
Assets subject to lien, amount	222,970	191,545
Other [Member]
Collateralized Transactions Assets Subject To Lien [Line Items]
Assets subject to lien, amount	¥ 25	¥ 30