Revenue from services provided to customers - Additional information (Detail) - JPY (¥) ¥ in Millions	12 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Mar. 31, 2017
Disaggregation of Revenue [Line Items]
Revenue - Commissions	¥ 293,069	¥ 373,313	¥ 327,129
Revenue from performance obligations satisfied in previous periods	1,334
Accounting Standards Update 2014-09 [Member]
Disaggregation of Revenue [Line Items]
Revenue - Commissions	¥ (17,297)

Amount of commission revenue from buying and selling securities on behalf of customers.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 940
-SubTopic 20
-Section 05
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=68070981&loc=d3e40544-110947

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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-SubTopic 220
-Section S99
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Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
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-URI http://asc.fasb.org/extlink&oid=109196051&loc=SL49130549-203045

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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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