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Fair value measurements - Geographic Allocations of Nomura's Trading Assets Related to Government, State, Municipal, and Government Agency Securities (Parenthetical) (Detail) (JPY ¥)
In Millions, unless otherwise specified
Sep. 30, 2014
Mar. 31, 2014
Fair value, concentration of credit risk [Line items]    
Non-trading debt securities ¥ 967,293us-gaap_AvailableForSaleSecuritiesDebtSecurities ¥ 1,023,746us-gaap_AvailableForSaleSecuritiesDebtSecurities
Government, agency and municipal securities [Member]    
Fair value, concentration of credit risk [Line items]    
Non-trading debt securities ¥ 681,000us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_FairValueConcentrationOfRiskDisclosureItemsAxis
= nmr_GovernmentAgencyAndMunicipalSecuritiesMember		 ¥ 756,000us-gaap_AvailableForSaleSecuritiesDebtSecurities
/ us-gaap_FairValueConcentrationOfRiskDisclosureItemsAxis
= nmr_GovernmentAgencyAndMunicipalSecuritiesMember