v3.25.2
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, 2025
Mar. 31, 2024
Mar. 31, 2023
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]      
Balance at beginning of fiscal year ¥ 18,517,087 ¥ 16,523,597  
Net change during the fiscal year 473,474 1,462,277 ¥ 598,005
Balance at end of fiscal year 19,249,738 18,517,087 16,523,597
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 2,237,625 871,054 283,550
Net change during the fiscal year 323,565 1,366,571 587,504
Balance at end of fiscal year 2,561,190 2,237,625 871,054
Net unrealized losses (gains) on investment securities      
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]      
Balance at beginning of fiscal year (826,271) (883,931) (676,100)
Net change during the fiscal year (56,790) 57,660 (207,831)
Balance at end of fiscal year (883,061) (826,271) (883,931)
Net debt valuation adjustments      
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]      
Balance at beginning of fiscal year (41,382) 3,268 (14,538)
Net change during the fiscal year 11,261 (44,650) 17,806
Balance at end of fiscal year (30,121) (41,382) 3,268
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 63 551 6,277
Net change during the fiscal year (1,262) (488) (5,726)
Balance at end of fiscal year (1,199) 63 551
Defined benefit plans      
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]      
Balance at beginning of fiscal year 389,392 8,350 28,946
Net change during the fiscal year 71,894 381,042 (20,596)
Balance at end of fiscal year 461,286 389,392 8,350
Foreign currency translation adjustments      
AOCI Including Portion Attributable to Noncontrolling Interest, Net of Tax [Roll Forward]      
Balance at beginning of fiscal year 2,715,823 1,742,816 938,965
Net change during the fiscal year 298,462 973,007 803,851
Balance at end of fiscal year ¥ 3,014,285 ¥ 2,715,823 ¥ 1,742,816