Komatsu Ltd. Shareholders' Equity [Details]	12 Months Ended
Mar. 31, 2011 USD ($)	Mar. 31, 2011 JPY (¥)	Mar. 31, 2010 USD ($)	Mar. 31, 2010 JPY (¥)	Mar. 31, 2009 JPY (¥)	Mar. 31, 2009
Stock option activity
Outstanding at beginning of year, Number of shares	3,333,000	3,333,000	2,891,000	2,891,000	2,844,000
Outstanding at beginning of year, Weighted average exercise price	$ 24.71	¥ 2,051		¥ 2,022	¥ 1,784
Granted, Number of Shares	76,800	76,800	642,000	642,000	463,000
Granted, Weighted average exercise price		¥ 1	$ 0.01	¥ 1,729	¥ 2,499
Exercised, Number of Shares	(174,000)	(174,000)	(200,000)	(200,000)	(416,000)
Exercised, Weighted average exercise price		¥ 1,238	$ 14.92	¥ 595	¥ 926
Outstanding at end of year, Number of shares	3,235,800	3,235,800	3,333,000	3,333,000
Outstanding at end of year, Weighted average exercise price	$ 24.66	¥ 2,047	$ 24.71	¥ 2,051	¥ 2,022
Exercisable at end of year, Number of Shares	3,159,000	3,159,000	2,691,000	2,691,000		2,428,000
Exercisable at end of year, Weighted average exercise price	$ 25.25	¥ 2,096		¥ 2,128	¥ 1,931

Share Based Compensation Arrangement By Share Based Payment Award OptionS Exercises In Period Weighted Average Exercise Price.

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Share Based Compensation Arrangement By Share Based Payment Award Options Grants In Period Weighted Average Exercise Price.

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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 Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph b(1)(c), d(2)

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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph b(1)(c)

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- Definition

The decrease in the number of reserved shares that could potentially be issued attributable to the exercise or conversion during the reporting period of previously issued stock options under the option plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph b(1)(e)

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- Definition

The quantity of shares issuable on stock options awarded under the plan during the reporting period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph b(1)(d)

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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph b(1)(a)

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph b(1)(b)

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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

The weighted average price as of the beginning of the year at which grantees can acquire the shares reserved for issuance under the stock option plan.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Statement of Financial Accounting Standard (FAS)
-Number 123R
-Paragraph A240
-Subparagraph b(1)(a)

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