Regulatory requirements - Additional information (Detail) ¥ in Millions	Mar. 31, 2022 JPY (¥)	Mar. 31, 2022 USD ($)	Mar. 31, 2021 JPY (¥)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity Tier 1 capital ratio	0.0751	0.0751
Tier 1 capital ratio	9.01%	9.01%
Consolidated Capital adequacy ratio	0.1101	0.1101
Quantified total of business risk	1.20	1.20	1.20
Segregated client cash recognized as an asset in Deposits with stock exchanges and other segregated cash \| ¥	¥ 113,052		¥ 92,477
Segregated securities recognized as assets in Trading assets and Collateralized agreements \| ¥	¥ 1,103,395		¥ 1,080,260
NSC [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Quantified total of business risk	1.20	1.20	1.20
NFPS [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Quantified total of business risk	1.20	1.20	1.20
NSI [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Ratio of net capital under the alternative method	2.00%	2.00%
Percentage of total risk margin requirement	8.00%	8.00%
Amount of maintenance of net capital as cash		$ 1,000,000
NGFP [Membrer]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Net capital defined under the alternative method		$ 20,000,000
Minimum net capital percentage of SEC risk margin amount	2.00%	2.00%
Minimum net capital percentage of CFTC risk margin amount	2.00%	2.00%
ILLC [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Net capital defined under the alternative method		$ 1,000,000
Ratio of net capital under the alternative method	2.00%	2.00%
Minimum capital required		$ 45,000

Minimum Net Capital Percentage of Aggregate Debit Items Arising from Client Transactions.

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Minimum net capital percentage of CFTC risk margin amount.

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Minimum net capital percentage of SEC risk margin amount.

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Minimum Net Capital Percentage of Total Risk Margin Requirement for all positions carried in client accounts and nonclient accounts.

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Tier One Capital Ratio

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Minimum amount of net capital required to be maintained by broker-dealer subsidiary to engage in security transaction, calculated under Aggregate Indebtedness Standard.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Rule 15c3-1
-Number 240
-Section 15c3-1

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Minimum amount of net capital required to be maintained by broker-dealer parent, calculated under Aggregate Indebtedness Standard.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher SEC
-Name Rule 15c3-1
-Number 240
-Section 15c3-1

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Amount of minimum total risk-based capital required for capital adequacy as defined by regulatory framework.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1B
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Ratio of minimum total risk-based capital to risk-weighted assets categorized as well capitalized as defined by regulatory framework for prompt corrective action.

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Ratio of total risk-based capital to risk-weighted assets as defined by regulatory framework.

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Cash deposited in a special reserve account for the exclusive benefit of customers pursuant to regulations other than SEC Regulations and the Commodity Exchange Act.

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Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=116631393&loc=SL116631396-227033

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Ratio of Common Equity Tier 1 risk-based capital to risk-weighted assets as defined by regulatory framework.

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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.

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Fair value of securities deposited in a special reserve account for the exclusive benefit of customers pursuant to regulations other than SEC Regulations and the Commodity Exchange Act.

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