Non-trading securities - Additional information (Detail) - Insurance subsidiary [Member] ¥ in Millions	3 Months Ended	6 Months Ended
Sep. 30, 2016 JPY (¥)	Sep. 30, 2015 JPY (¥)	Sep. 30, 2016 JPY (¥)	Sep. 30, 2015 JPY (¥)	Mar. 31, 2016
Non-trading debt securities [Line Items]
Non-trading securities disposed	¥ 21,156	¥ 24,603	¥ 34,986	¥ 33,884
Non-trading securities realized gain	2,435	3,005	3,353	3,490
Non-trading securities realized loss	727	20	1,064	138
Total proceeds received from disposals	¥ 22,864	27,588	¥ 37,275	37,236
Total number of non-trading securities that are in an unrealized loss positions	63		63		52
Other-than-temporary impairment loss recognized for non-trading equity securities	¥ 271	433	¥ 872	433
Credit loss component of other-than-temporary impairment losses recognized for non-trading debt securities	¥ 24		¥ 210
Other-than-temporary impairment loss recognized within other comprehensive income (loss)		¥ (27)		¥ 20

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.

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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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The non-credit loss component of other-than-temporary impairment losses recognized for Government, agency and municipal securities and other debt securities and reported within Other comprehensive income (loss) .

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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=73527792&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=73527792&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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