Commitments, contingencies and guarantees - Information on derivative contracts and standby letters of credit and other guarantees (Parenthetical) (Detail) - JPY (¥) ¥ in Millions	Mar. 31, 2019	Mar. 31, 2018
Commitments, Contingencies and Guarantees [Abstract]
Collateral held in connection with standby letters of credit and other guarantees	¥ 2,481	¥ 2,559

Collateral held in connection with standby letters of credit and other guarantees.

No definition available.

Name:	nmr_CollateralHeldInConnectionWithGuarantees
Namespace Prefix:	nmr_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant