v3.26.1
Severance Indemnities and Pension Plans (Summary of Assumptions Used in Computation) (Detail)
 12 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Mar. 31, 2024
Domestic, Japan | Pension benefits and SIPs      
Defined Benefit Plan Disclosure [Line Items]      
Discount rates in determining expense 2.89% 1.70% 1.36%
Discount rates in determining benefit obligation 3.33% 2.89% 1.70%
Rates of increase in future compensation level for determining expense 3.91% 3.49% 3.47%
Rates of increase in future compensation level for determining benefit obligation 2.76% 3.91% 3.49%
Expected rates of return on plan assets 2.91% 2.89% 2.97%
Cash balance crediting rate for determining expense 2.53% 2.46% 2.46%
Cash balance crediting rate for determining benefit obligation 3.15% 2.53% 2.46%
Foreign offices and subsidiaries | Pension benefits      
Defined Benefit Plan Disclosure [Line Items]      
Discount rates in determining expense 5.17% 5.06% 4.74%
Discount rates in determining benefit obligation 5.39% 5.22% 5.06%
Rates of increase in future compensation level for determining expense 5.44% 5.50% 5.09%
Rates of increase in future compensation level for determining benefit obligation 5.41% 5.44% 5.50%
Expected rates of return on plan assets 5.80% 5.93% 4.98%
Cash balance crediting rate for determining expense 4.59% 4.38% 3.72%
Cash balance crediting rate for determining benefit obligation 4.88% 4.59% 4.38%
Foreign offices and subsidiaries | Other benefits      
Defined Benefit Plan Disclosure [Line Items]      
Discount rates in determining expense 4.89% 5.00% 4.59%
Discount rates in determining benefit obligation 5.45% 5.26% 5.06%
Rates of increase in future compensation level for determining expense 0.00% 0.00% 0.00%
Rates of increase in future compensation level for determining benefit obligation 0.00% 0.00% 0.00%
Expected rates of return on plan assets 6.50% 7.00% 6.25%
Cash balance crediting rate for determining expense 0.00% 0.00% 0.00%
Cash balance crediting rate for determining benefit obligation 0.00% 0.00% 0.00%