v3.26.1
Severance Indemnities and Pension Plans (Components of Net Periodic Cost of Pension Benefits and Other Benefits) (Detail) - JPY (¥)
¥ in Millions
 12 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Mar. 31, 2024
Defined Benefit Plan Disclosure [Line Items]      
Net periodic benefit cost (income) ¥ (76,990) ¥ (63,046) ¥ (26,771)
Domestic, Japan | Pension benefits and SIPs      
Defined Benefit Plan Disclosure [Line Items]      
Service cost—benefits earned during the fiscal year 28,099 31,728 34,653
Interest cost on projected benefit obligation 30,210 23,954 20,547
Expected return on plan assets (100,224) (95,929) (83,130)
Amortization of net actuarial loss (gain) (30,905) (16,821) (591)
Amortization of prior service cost 147 (282) (1,974)
Loss (gain) on settlements and curtailment (15,807) (13,754) (13,659)
Other 141 (203) (194)
Net periodic benefit cost (income) (88,339) (71,307) (44,348)
Foreign offices and subsidiaries | Pension benefits      
Defined Benefit Plan Disclosure [Line Items]      
Service cost—benefits earned during the fiscal year 13,354 14,759 13,231
Interest cost on projected benefit obligation 9,303 8,850 8,461
Expected return on plan assets (12,039) (12,443) (10,810)
Amortization of net actuarial loss (gain) 2,243 2,112 1,362
Amortization of prior service cost (420) (1,328) (945)
Loss (gain) on settlements and curtailment (418) 15 1,803
Other 0 (2,513) 3,939
Net periodic benefit cost (income) 12,023 9,452 17,041
Foreign offices and subsidiaries | Other benefits      
Defined Benefit Plan Disclosure [Line Items]      
Service cost—benefits earned during the fiscal year 77 52 52
Interest cost on projected benefit obligation 1,144 1,266 1,216
Expected return on plan assets (1,474) (1,670) (1,503)
Amortization of net actuarial loss (gain) 23 (89) 1,191
Amortization of prior service cost (444) (449) (426)
Loss (gain) on settlements and curtailment 0 0 0
Other 0 (301) 6
Net periodic benefit cost (income) ¥ (674) ¥ (1,191) ¥ 536