Commitments and contingencies - Schedule of undrawn commitment (Details) - JPY (¥) ¥ in Millions	Mar. 31, 2026	Mar. 31, 2025
Disclosure of detailed information about borrowings [line items]
Balance of executed borrowed commitment line	¥ 2,758	¥ 751
Monex Finance Corporation and JSF Trust and Banking Co., Ltd
Disclosure of detailed information about borrowings [line items]
Total amount of commitment line borrowing		8,900
Balance of executed borrowed commitment line		751
Balance of undrawn commitment line		¥ 8,149
Monex Finance Corporation, JSF Trust and Banking Co., Ltd. and Aozora Bank, Ltd.
Disclosure of detailed information about borrowings [line items]
Total amount of commitment line borrowing	11,054
Balance of executed borrowed commitment line	2,524
Balance of undrawn commitment line	¥ 8,530

The amount of outstanding funds that the entity is obligated to repay.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
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- Definition

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

+ References

No definition available.

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- Definition

The nominal or face amount of a financial instrument, used to calculate payments made on that instrument.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Note Expiry date 2027-01-01
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- Definition

The amount of undrawn borrowing facilities that may be available for future operating activities and to settle capital commitments. [Refer: Capital commitments]

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
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