v3.26.1
Accumulated Other Comprehensive Income (Changes in Accumulated OCI, Net of Tax and Net of Noncontrolling Interests) (Detail) - JPY (¥)
¥ in Millions
 12 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Mar. 31, 2024
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]      
Balance at beginning of fiscal year ¥ 19,249,738 ¥ 18,517,087  
Net change during the fiscal year 909,432 473,474 ¥ 1,462,277
Balance at end of fiscal year 20,840,057 19,249,738 18,517,087
Accumulated other comprehensive income, net of taxes:      
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]      
Balance at beginning of fiscal year 2,561,190 2,237,625 871,054
Net change during the fiscal year 899,261 323,565 1,366,571
Balance at end of fiscal year 3,460,451 2,561,190 2,237,625
Net unrealized losses on investment securities      
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]      
Balance at beginning of fiscal year (883,061) (826,271) (883,931)
Net change during the fiscal year 39,983 (56,790) 57,660
Balance at end of fiscal year (843,078) (883,061) (826,271)
Net debt valuation adjustments      
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]      
Balance at beginning of fiscal year (30,121) (41,382) 3,268
Net change during the fiscal year 2,818 11,261 (44,650)
Balance at end of fiscal year (27,303) (30,121) (41,382)
Net unrealized gains (losses) on derivatives qualifying for cash flow hedges:      
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]      
Balance at beginning of fiscal year (1,199) 63 551
Net change during the fiscal year (6,263) (1,262) (488)
Balance at end of fiscal year (7,462) (1,199) 63
Defined benefit plans      
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]      
Balance at beginning of fiscal year 461,286 389,392 8,350
Net change during the fiscal year 213,812 71,894 381,042
Balance at end of fiscal year 675,098 461,286 389,392
Foreign currency translation adjustments      
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]      
Balance at beginning of fiscal year 3,014,285 2,715,823 1,742,816
Net change during the fiscal year 648,911 298,462 973,007
Balance at end of fiscal year ¥ 3,663,196 ¥ 3,014,285 ¥ 2,715,823