Income Taxes - Additional Information (Detail) - JPY (¥) ¥ in Millions	12 Months Ended
Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Income Taxes [Line Items]
Net operating loss carryforwards	¥ 637,000
Deferred tax liabilities	(110,839)	¥ 58,345
Changes that directly affected Income tax expense	5,984	(2,444)	¥ (9,102)
Unrecognized tax benefits, that would affect effective tax rate	3,374	12,323	2,345
Unrecognized tax benefits, interest and penalties	1,477	¥ 2,164	¥ 888
Mizuho Securities Company Limited \| Japan
Income Taxes [Line Items]
Net operating loss carryforwards	333,000
Mizuho Financial Group Inc
Income Taxes [Line Items]
Undistributed earnings	220,000
Deferred tax liabilities	¥ 21,000

Deferred Tax Assets Valuation Allowance, Changes Directly Affected Deferred Tax Expense

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- Definition

Income Taxes [Line Items]

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Amount, after deferred tax asset, of deferred tax liability attributable to taxable differences without jurisdictional netting.

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Amount of operating loss carryforward, before tax effects, available to reduce future taxable income under enacted tax laws.

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The earnings that is allocated to common stock and participating securities to the extent that each security may share in earnings as if all of the earnings for the period had been distributed.

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Amount accrued for interest on an underpayment of income taxes and penalties related to a tax position claimed or expected to be claimed in the tax return.

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The total amount of unrecognized tax benefits that, if recognized, would affect the effective tax rate.

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