v3.26.1
Severance Indemnities and Pension Plans (Weighted-average Target Asset Allocation of Plan Assets for Pension Benefits and Other Benefits) (Detail)
Mar. 31, 2026
Domestic, Japan | Pension benefits and SIPs  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 100.00%
Domestic, Japan | Pension benefits and SIPs | Japanese equity securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 26.30%
Domestic, Japan | Pension benefits and SIPs | Japanese debt securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 22.50%
Domestic, Japan | Pension benefits and SIPs | Non-Japanese equity securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 14.20%
Domestic, Japan | Pension benefits and SIPs | Non-Japanese debt securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 22.10%
Domestic, Japan | Pension benefits and SIPs | Real estate  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 1.30%
Domestic, Japan | Pension benefits and SIPs | Short-term assets and other  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 13.60%
Foreign offices and subsidiaries | Pension benefits  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 100.00%
Foreign offices and subsidiaries | Pension benefits | Japanese equity securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 0.00%
Foreign offices and subsidiaries | Pension benefits | Japanese debt securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 0.00%
Foreign offices and subsidiaries | Pension benefits | Non-Japanese equity securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 30.20%
Foreign offices and subsidiaries | Pension benefits | Non-Japanese debt securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 57.40%
Foreign offices and subsidiaries | Pension benefits | Real estate  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 7.60%
Foreign offices and subsidiaries | Pension benefits | Short-term assets and other  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 4.80%
Foreign offices and subsidiaries | Other benefits  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 100.00%
Foreign offices and subsidiaries | Other benefits | Japanese equity securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 0.00%
Foreign offices and subsidiaries | Other benefits | Japanese debt securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 0.00%
Foreign offices and subsidiaries | Other benefits | Non-Japanese equity securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 30.00%
Foreign offices and subsidiaries | Other benefits | Non-Japanese debt securities  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 64.00%
Foreign offices and subsidiaries | Other benefits | Real estate  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 6.00%
Foreign offices and subsidiaries | Other benefits | Short-term assets and other  
Defined Benefit Plan Disclosure [Line Items]  
Target allocation of plan assets 0.00%