v3.26.1
Investment Securities - Summary of Investment Securities (Detail) - JPY (¥)
¥ in Millions
Mar. 31, 2026
Mar. 31, 2025
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at amortized cost ¥ 4,763,716 ¥ 366,997
Equity instruments at fair value through other comprehensive income 5,505,439 5,170,856
Debt instruments at fair value through other comprehensive income 23,268,702 28,008,280
Total investment securities 33,537,857 33,546,133
Domestic [member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at amortized cost 4,585,015 274,414
Debt instruments at fair value through other comprehensive income 6,653,719 12,649,244
Domestic [member] | Japanese government bonds [member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at amortized cost 4,420,130 109,550
Domestic [member] | Japanese municipal bonds [member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at amortized cost 151,898 151,882
Domestic [member] | Japanese corporate bonds [member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at amortized cost 12,987 12,982
Domestic [member] | Debt instruments [member] | Japanese government bonds [member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at fair value through other comprehensive income 5,476,428 11,180,546
Domestic [member] | Debt instruments [member] | Japanese municipal bonds [member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at fair value through other comprehensive income 683,035 822,575
Domestic [member] | Debt instruments [member] | Japanese corporate bonds [member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at fair value through other comprehensive income 493,734 645,602
Domestic [member] | Debt instruments [member] | Other debt instruments [member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at fair value through other comprehensive income 522 521
Foreign [member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at amortized cost 178,701 92,583
Debt instruments at fair value through other comprehensive income 16,614,983 15,359,036
Foreign [member] | Bonds issued by governments and official institutions excluding U.S. Treasury and other U.S. government agency bonds [Member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at amortized cost 83,011 66,896
Foreign [member] | Mortgage-backed securities [member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at amortized cost 14,933 13,876
Foreign [member] | Other debt instruments [member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at amortized cost 80,757 11,811
Foreign [member] | Debt instruments [member] | U.S. Treasury and other U.S. government agency bonds [member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at fair value through other comprehensive income 6,809,035 5,539,051
Foreign [member] | Debt instruments [member] | Bonds issued by governments and official institutions excluding U.S. Treasury and other U.S. government agency bonds [Member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at fair value through other comprehensive income 5,314,924 4,531,619
Foreign [member] | Debt instruments [member] | Mortgage-backed securities [member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at fair value through other comprehensive income 3,392,908 4,238,051
Foreign [member] | Debt instruments [member] | Other debt instruments [member]    
Disclosure of detailed information about financial instruments [line items]    
Debt instruments at fair value through other comprehensive income 1,098,116 1,050,315
Domestic equity instruments [member]    
Disclosure of detailed information about financial instruments [line items]    
Equity instruments at fair value through other comprehensive income 3,840,059 3,362,996
Foreign equity instruments [member]    
Disclosure of detailed information about financial instruments [line items]    
Equity instruments at fair value through other comprehensive income ¥ 1,665,380 ¥ 1,807,860