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Income Taxes - Additional Information (Detail)
3 Months Ended 6 Months Ended
Sep. 30, 2014
Sep. 30, 2013
Sep. 30, 2014
Sep. 30, 2013
Income Taxes [Line Items]        
Domestic effective statutory tax rate 36.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 38.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 36.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate 38.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
Effective tax rate 28.30%us-gaap_EffectiveIncomeTaxRateContinuingOperations 47.40%us-gaap_EffectiveIncomeTaxRateContinuingOperations 40.80%us-gaap_EffectiveIncomeTaxRateContinuingOperations 43.80%us-gaap_EffectiveIncomeTaxRateContinuingOperations
Between April 1 2012 to March 31 2014 [Member]        
Income Taxes [Line Items]        
Domestic effective statutory tax rate 36.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
/ nmr_IncomeTaxAssesmentPeriodAxis
= nmr_BetweenApril12012ToMarch312015Member
  36.00%us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate
/ nmr_IncomeTaxAssesmentPeriodAxis
= nmr_BetweenApril12012ToMarch312015Member