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Derivative instruments and hedging activities - Concentration of Exposures to Credit Risk in OTC Derivatives (Detail) (JPY ¥)
In Billions, unless otherwise specified
Sep. 30, 2014
Mar. 31, 2014
Derivative [Line Items]    
Gross fair value of derivative assets, Financial institutions ¥ 32,150us-gaap_DerivativeFairValueOfDerivativeAsset ¥ 26,069us-gaap_DerivativeFairValueOfDerivativeAsset
OTC derivatives with financial institutions [Member]    
Derivative [Line Items]    
Gross fair value of derivative assets, Financial institutions 25,615us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_InvestmentTypeAxis
= nmr_OtcDerivativesWithFinancialInstitutionsMember		 20,355us-gaap_DerivativeFairValueOfDerivativeAsset
/ us-gaap_InvestmentTypeAxis
= nmr_OtcDerivativesWithFinancialInstitutionsMember
Impact of master netting agreements, Financial institutions (23,476)nmr_ImpactOfMasterNettingArrangements
/ us-gaap_InvestmentTypeAxis
= nmr_OtcDerivativesWithFinancialInstitutionsMember		 (18,481)nmr_ImpactOfMasterNettingArrangements
/ us-gaap_InvestmentTypeAxis
= nmr_OtcDerivativesWithFinancialInstitutionsMember
Impact of collateral, Financial institutions (1,246)nmr_ImpactOfCollateralExposureToDerivatives
/ us-gaap_InvestmentTypeAxis
= nmr_OtcDerivativesWithFinancialInstitutionsMember		 (936)nmr_ImpactOfCollateralExposureToDerivatives
/ us-gaap_InvestmentTypeAxis
= nmr_OtcDerivativesWithFinancialInstitutionsMember
Net exposure to credit risk, Financial institutions ¥ 893nmr_NetExposureToCreditRiskFromDerivatives
/ us-gaap_InvestmentTypeAxis
= nmr_OtcDerivativesWithFinancialInstitutionsMember		 ¥ 938nmr_NetExposureToCreditRiskFromDerivatives
/ us-gaap_InvestmentTypeAxis
= nmr_OtcDerivativesWithFinancialInstitutionsMember