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Non-trading investments - Summary of AFS debt securities in an unrealized loss position (Detail)
¥ in Millions
Sep. 30, 2025
JPY (¥)
SECURITIES
Mar. 31, 2025
JPY (¥)
SECURITIES
Fair value    
Total ¥ 193,508 ¥ 94,656
Unrealized losses    
Total ¥ (2,131) ¥ (1,693)
Number of debt securities    
Total | SECURITIES 143 79
Japanese government securities [Member]    
Fair value    
Less than 12 months ¥ 59,864 ¥ 24,814
12 months or longer 4,896
Total 64,760 24,814
Unrealized losses    
Less than 12 months (138) (187)
12 months or longer (110)
Total ¥ (248) ¥ (187)
Number of debt securities    
Less than 12 months | SECURITIES 7 4
12 months or longer | SECURITIES 1
Total | SECURITIES 8 4
Japanese agency and municipal securities [Member]    
Fair value    
Less than 12 months ¥ 108,024 ¥ 68,063
12 months or longer 11,125
Total 119,149 68,063
Unrealized losses    
Less than 12 months (1,546) (1,495)
12 months or longer (268)
Total ¥ (1,814) ¥ (1,495)
Number of debt securities    
Less than 12 months | SECURITIES 103 72
12 months or longer | SECURITIES 15
Total | SECURITIES 118 72
Bank and corporate debt securities [Member]    
Fair value    
Less than 12 months ¥ 9,599 ¥ 1,779
Total 9,599 1,779
Unrealized losses    
Less than 12 months (69) (11)
Total ¥ (69) ¥ (11)
Number of debt securities    
Less than 12 months | SECURITIES 17 3
Total | SECURITIES 17 3