Non-trading securities - Additional information (Detail) - Insurance subsidiary [Member] ¥ in Millions	3 Months Ended	6 Months Ended
Sep. 30, 2017 JPY (¥)	Sep. 30, 2016 JPY (¥)	Sep. 30, 2017 JPY (¥)	Sep. 30, 2016 JPY (¥)	Mar. 31, 2017
Non-trading debt securities [Line Items]
Non-trading securities disposed	¥ 8,683	¥ 21,156	¥ 14,398	¥ 34,986
Non-trading securities realized gains	274	2,435	393	3,353
Non-trading securities realized losses	365	727	365	1,064
Total proceeds received from disposals	¥ 8,592	22,864	¥ 14,426	37,275
Total number of non-trading securities that are in an unrealized loss positions	38		38		41
Other-than-temporary impairment losses recognized for non-trading equity securities	¥ 19	271	¥ 19	872
Credit loss component of other-than-temporary impairment losses recognized for non-trading debt securities		¥ 24	¥ 29	¥ 210

Available For Sale Securities Continuous Unrealized Loss Position Number Of Securities

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- Definition

For any sales of securities categorized neither as held-to-maturity nor trading, this item represents the net carrying amount of the sold security.

+ References

No definition available.

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This item represents the gain realized on the sale of securities categorized neither as held-to-maturity nor trading disposed of during the period.

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This item represents the loss realized on the sale of securities categorized neither as held-to-maturity nor trading disposed of during the period.

+ References

No definition available.

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- Definition

The credit loss component of other-than-temporary impairment losses recognized for non-trading debt securities

+ References

No definition available.

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- Definition

The total amount of proceeds received for the sale of securities, categorized neither as held-to-maturity nor trading securities, during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 9
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563

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- Definition

The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of its amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 8B
-URI http://asc.fasb.org/extlink&oid=82897076&loc=SL6284393-111563

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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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