Collateralized transactions - Securities transferred in repurchase agreements and securities lending transactions (Parenthetical) (Detail) - JPY (¥) ¥ in Billions	Sep. 30, 2018	Mar. 31, 2018
Collateralized Transactions
U.S. government sponsored agency mortgage pass-through securities and collateralized mortgage obligations	¥ 4,752	¥ 3,351

US government sponsored agency mortgage pass-through securities and collateralized mortgage obligations

No definition available.

Name:	nmr_USGovernmentSponsoredAgencyMortgagePassThroughSecuritiesCollateralizedMortgageObligations
Namespace Prefix:	nmr_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant