v3.26.1
Investments - Additional Information (Detail) - JPY (¥)
¥ in Millions
Mar. 31, 2026
Mar. 31, 2025
Gain (Loss) on Securities [Line Items]    
Equity method Investments, carrying value ¥ 978,287 ¥ 907,413
Orient Corporation    
Gain (Loss) on Securities [Line Items]    
Percentage of proportionate share to the total outstanding common stock 48.96%  
Joint Stock Commercial Bank Foreign Trade of Vietnam    
Gain (Loss) on Securities [Line Items]    
Percentage of proportionate share to the total outstanding common stock 15.00%  
Mizuho Leasing Company LTD    
Gain (Loss) on Securities [Line Items]    
Percentage of proportionate share to the total outstanding common stock 23.54%  
Custody Bank of Japan Ltd    
Gain (Loss) on Securities [Line Items]    
Percentage of proportionate share to the total outstanding common stock 27.00%  
Rakuten Securities, Inc    
Gain (Loss) on Securities [Line Items]    
Percentage of proportionate share to the total outstanding common stock 49.00%  
Japan Investor Solutions & Technologies Co., Ltd. [Member]    
Gain (Loss) on Securities [Line Items]    
Percentage of proportionate share to the total outstanding common stock 39.04%  
Equity securities (marketable)    
Gain (Loss) on Securities [Line Items]    
Equity method Investments, carrying value ¥ 377,044 340,574
Equity method Investments, aggregated market values ¥ 617,026 ¥ 452,117