Deferred compensation plans - Activity Relating to SAR Plan A (Detail) (SAR Plan A [Member], JPY ¥)	12 Months Ended
Mar. 31, 2013	Mar. 31, 2012
SAR Plan A [Member]
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Outstanding (number of shares), Outstanding, Beginning	15,354,300
Outstanding (number of shares), Granted	2,847,800
Outstanding (number of shares), Exercised	(18,000)
Outstanding (number of shares), Forfeited	(171,000)
Outstanding (number of shares), Expired	(1,476,800)
Outstanding (number of shares), Outstanding, Ending	16,545,500	15,354,300
Outstanding (number of shares), Exercisable, Ending	10,880,700
Weighted-average exercise price, Outstanding, Beginning	¥ 988
Weighted-average exercise price, Granted	¥ 298
Weighted-average exercise price, Exercised	¥ 478
Weighted-average exercise price, Forfeited	¥ 896
Weighted-average exercise price, Expired	¥ 1,120
Weighted-average exercise price, Outstanding, Ending	¥ 848	¥ 988
Weighted-average exercise price, Exercisable, Ending	¥ 1,134
Weighted-average remaining life (years), Outstanding	3 years 10 months	3 years 11 months
Weighted-average remaining life (years), Exercisable	2 years 8 months

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

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.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 718

-SubTopic 10

-Section 50

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- Definition

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.

+ References

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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.

+ References

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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.

+ References

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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.

+ References

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-Publisher FASB

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-Topic 718

-SubTopic 10

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-Paragraph 2

-Subparagraph (c)(1)(iv)(3)

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- Definition

The number of shares reserved for issuance under stock option agreements awarded under the plan that validly exist and are outstanding as of the balance sheet date, including vested options.

+ References

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-Paragraph A240

-Subparagraph b(1)(b)

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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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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.

+ References

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- Definition

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

+ References

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.

+ References

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

+ References

No definition available.

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- Definition

Weighted average price at which grantees can acquire the shares reserved for issuance on stock options awarded.

+ References

No definition available.

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- Definition

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

+ References

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