Derivative Financial Instruments (Summary of Gains and Losses Information Related to Net Investment Hedges) (Parenthetical) (Detail) - JPY (¥) ¥ in Millions	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Financial instruments hedging foreign exchange risk
Derivative Instruments, Gain (Loss) [Line Items]
Accumulated other comprehensive income reclassified to earnings	¥ (1,336)	¥ 186	¥ 0

The effective portion of gains and losses on derivatives and nonderivative financial instruments designated and qualifying in net investment hedges recorded in accumulated other comprehensive income (loss) during the term of the hedging relationship and reclassified into earnings during the current period.

No definition available.

Name:	mfg_AccumulatedOtherComprehensiveIncomeLossEffectivePortionUnderNetInvestmentHedgeReclassifiedToEarningsDuringPeriod
Namespace Prefix:	mfg_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant