Securitizations and Variable Interest Entities - Carrying amount of variable interests of unconsolidated VIEs and maximum exposure to loss (Detail) - JPY (¥) ¥ in Billions	Mar. 31, 2020	Mar. 31, 2019
Variable Interest Entity [Line Items]
Carrying amount of variable interests, Assets	¥ 4,766	¥ 3,561
Carrying amount of variable interests, Liabilities
Maximum exposure to loss to unconsolidated VIEs	4,850	3,645
Equities [Member]
Variable Interest Entity [Line Items]
Carrying amount of variable interests, Assets	35	29
Carrying amount of variable interests, Liabilities
Maximum exposure to loss to unconsolidated VIEs	35	29
Debt securities [Member]
Variable Interest Entity [Line Items]
Carrying amount of variable interests, Assets	73	109
Carrying amount of variable interests, Liabilities
Maximum exposure to loss to unconsolidated VIEs	73	109
CMBS and RMBS [Member]
Variable Interest Entity [Line Items]
Carrying amount of variable interests, Assets	3,631	2,654
Carrying amount of variable interests, Liabilities
Maximum exposure to loss to unconsolidated VIEs	3,631	2,654
Investment trust funds and other [Member]
Variable Interest Entity [Line Items]
Carrying amount of variable interests, Assets	170	153
Carrying amount of variable interests, Liabilities
Maximum exposure to loss to unconsolidated VIEs	170	153
Private equity and debt investments [Member]
Variable Interest Entity [Line Items]
Carrying amount of variable interests, Assets	11	12
Carrying amount of variable interests, Liabilities
Maximum exposure to loss to unconsolidated VIEs	11	12
Loans [Member]
Variable Interest Entity [Line Items]
Carrying amount of variable interests, Assets	835	593
Carrying amount of variable interests, Liabilities
Maximum exposure to loss to unconsolidated VIEs	835	593
Other [Member]
Variable Interest Entity [Line Items]
Carrying amount of variable interests, Assets	11	11
Carrying amount of variable interests, Liabilities
Maximum exposure to loss to unconsolidated VIEs	11	11
Commitments to extend credit and other guarantees [Member]
Variable Interest Entity [Line Items]
Carrying amount of variable interests, Assets
Carrying amount of variable interests, Liabilities
Maximum exposure to loss to unconsolidated VIEs	¥ 84	¥ 84

The reporting entity's maximum amount of exposure to loss as a result of its involvement with the Variable Interest Entity (VIE).

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5747-111685

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph b
-URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5728-111685

+ Details

Name:	us-gaap_VariableInterestEntityEntityMaximumLossExposureAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_VariableInterestEntityLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Carrying amount of the assets in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph a
-URI http://asc.fasb.org/extlink&oid=118582676&loc=d3e5728-111685

+ Details

Name:	us-gaap_VariableInterestEntityNonconsolidatedCarryingAmountAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Carrying amount of the liabilities in the reporting entity's statement of financial position that relate to the reporting entity's variable interest in the Variable Interest Entity (VIE), where the reporting entity is not the VIE's primary beneficiary.

+ References

+ Details

Name:	us-gaap_VariableInterestEntityNonconsolidatedCarryingAmountLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_EquitySecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_DebtSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_MortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=nmr_InvestmentTrustFundsAndOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=nmr_PrivateEquityAndDebtInvestmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_LoansReceivableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=nmr_OtherAssetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis=us-gaap_CommitmentsToExtendCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: