v3.26.1
Accumulated other comprehensive income (loss), net of tax (Amounts Reclassified Out of Accumulated Other Comprehensive Income into Net Income) (Detail)
¥ in Millions
 12 Months Ended
Mar. 31, 2026
JPY (¥)
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]  
Before tax ¥ (32,237) [1]
Tax effect 12,020 [2]
Net of tax before allocation to noncontrolling interests (20,217)
Net of tax attributable to noncontrolling interests 2 [2]
Net of tax attributable to MHFG shareholders (20,215)
Accumulated Net unrealized gains (losses) on available-for-sale securities | Investment gains (losses) net  
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]  
Before tax (79,123) [1]
Tax effect 24,560 [2]
Net of tax before allocation to noncontrolling interests (54,563)
Net of tax attributable to noncontrolling interests 0 [2]
Net of tax attributable to MHFG shareholders (54,563)
Accumulated Foreign currency translation adjustments | Foreign exchange gains (losses)—net Other noninterest income  
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]  
Before tax 6,566 [1]
Tax effect 0 [2]
Net of tax before allocation to noncontrolling interests 6,566
Net of tax attributable to noncontrolling interests 0 [2]
Net of tax attributable to MHFG shareholders 6,566
Accumulated Defined Benefit Plans Adjustments | Salaries and employee benefits  
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]  
Before tax 40,220 [1]
Tax effect (12,509) [2]
Net of tax before allocation to noncontrolling interests 27,711
Net of tax attributable to noncontrolling interests 2 [2]
Net of tax attributable to MHFG shareholders 27,714
Accumulated Own Credit Risk Adjustments | Other Noninterest Income (Expenses)  
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]  
Before tax 100 [1]
Tax effect (32) [2]
Net of tax before allocation to noncontrolling interests 69
Net of tax attributable to noncontrolling interests 0 [2]
Net of tax attributable to MHFG shareholders ¥ 69
[1] The financial statement line item in which the amounts in the before tax column are reported in the consolidated statements of income is listed to the right of the table.
[2] The financial statement line items in which the amounts in the tax effect and the net of tax attributable to noncontrolling interest columns are reported in the consolidated statements of income are Income tax expense (benefit) and Net income (loss), respectively.