Segment Information - Additional Information (Detail) ¥ in Millions	12 Months Ended
Mar. 31, 2018 JPY (¥)
Corporate Financial Services
Segment Reporting Information [Line Items]
Revenues	¥ 3
Investment Banking
Segment Reporting Information [Line Items]
Revenues	¥ 320,446

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.

No definition available.

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