Deferred compensation awards - Activity relating to SAR Plan A awards (Detail) - SAR Plan A awards [Member] - ¥ / shares	12 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Outstanding
Outstanding (number of shares), Beginning	16,539,300
Outstanding (number of shares), Granted
Outstanding (number of shares), Exercised	(900,800)
Outstanding (number of shares), Forfeited	(89,900)
Outstanding (number of shares), Expired	(95,700)
Outstanding (number of shares), Ending	15,452,900	16,539,300
Outstanding (number of shares), Exercisable, Ending	12,945,000
Weighted-average exercise price
Weighted-average exercise price, Beginning	¥ 679
Weighted-average exercise price, Granted
Weighted-average exercise price, Exercised	298
Weighted-average exercise price, Forfeited	630
Weighted-average exercise price, Expired	298
Weighted-average exercise price, Ending	704	¥ 679
Weighted-average exercise price, Exercisable, Ending	¥ 729
Weighted-average remaining life until expiry (years)
Weighted-average remaining life until expiry (years), Outstanding	3 years 1 month 6 days	3 years 10 months 24 days
Weighted-average remaining life until expiry (years), Exercisable	2 years 7 months 6 days

No definition available.

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- Definition

The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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- Definition

Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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- Definition

The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

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- Definition

Gross number of share options (or share units) granted during the period.

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No definition available.

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- Definition

Number of options outstanding, including both vested and non-vested options.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

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- Definition

Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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- Definition

Weighted average price at which option holders acquired shares when converting their stock options into shares.

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No definition available.

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- Definition

Weighted average price at which grantees could have acquired the underlying shares with respect to stock options of the plan that expired.

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No definition available.

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- Definition

Weighted average price at which grantees could have acquired the underlying shares with respect to stock options that were terminated.

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No definition available.

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- Definition

Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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No definition available.

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- Definition

Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Number of share options (or share units) exercised during the current period.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
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