Regulatory requirements - Additional information (Detail) ¥ in Millions	Mar. 31, 2017 JPY (¥)	Mar. 31, 2017 USD ($)	Mar. 31, 2016 JPY (¥)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity Tier 1 capital ratio	6.00%	6.00%
Tier 1 capital ratio	7.50%	7.50%
Capital adequacy ratio	9.50%	9.50%
Quantified total of business risk	120.00%	120.00%
Segregated client cash recognized as an asset in Deposits with stock exchanges and other segregated cash \| ¥	¥ 94,483		¥ 96,887
Segregated securities recognized as assets in Trading assets and Collateralized agreements \| ¥	¥ 768,616		¥ 526,979
NSC [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Quantified total of business risk	120.00%	120.00%	120.00%
NFPS [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Quantified total of business risk	120.00%	120.00%	120.00%
NSI [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%
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
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

Common Equity Tier One Capital Ratio

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- Definition

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

+ References

No definition available.

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- Definition

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

+ References

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- Definition

Tier One Capital Ratio

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No definition available.

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- Definition

The minimum amount of Total Capital required for capital adequacy purposes under the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1H
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=35709795&loc=d3e65071-112826

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826

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- Definition

The Total risk based capital ratio (total capital divided by risk weighted assets) required to be categorized as "well capitalized", in the regulatory framework for prompt corrective action.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=35709795&loc=SL5958568-112826

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- Definition

Total Capital divided by risk weighted assets as defined in the regulations.

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- Definition

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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 305
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6478312&loc=d3e61473-112792

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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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- Definition

Minimum amount of net capital required to be maintained by entity's broker-dealer subsidiary to engage in securities transactions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Rule 15c3-1
-Number 240
-Section 15c3-1

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- Definition

Minimum amount of net capital required to be maintained by the entity to engage in securities transactions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher SEC
-Name Rule 15c3-1
-Number 240
-Section 15c3-1

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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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