Loans - Credit Quality Information of Loans Based on MHFG Group's Internal Rating System (Parenthetical) (Detail) $ in Millions	12 Months Ended
Mar. 31, 2020 USD ($)
Financing Receivables Impaired [Member] \| Foreign [Member] \| Sale Of Impaired Loans And Credit Losses [Member]
Accounts, Notes, Loans and Financing Receivable [Line Items]
Decrease in impaired loans	$ 15,960

The increase (decrease) during the reporting period in outstanding loans including accrued interest.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

Name:	us-gaap_IncreaseDecreaseInFinanceReceivables
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration