v3.26.1
Fair Value (Quantitative Information about Level 3 Fair Value Measurements) (Detail) - Fair value, recurring measurements - Level 3
¥ in Millions
Mar. 31, 2026
JPY (¥)
Mar. 31, 2025
JPY (¥)
Interest rate contracts | Option model    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Fair value ¥ (27,506) ¥ (21,934)
Interest rate contracts | Option model | Recovery rate | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs   0.800
Interest rate contracts | Option model | Recovery rate | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs   0.900
Interest rate contracts | Option model | Recovery rate | Median    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs   0.850
Interest rate contracts | Option model | Correlation between interest rates | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.341 0.300
Interest rate contracts | Option model | Correlation between interest rates | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.572 0.604
Interest rate contracts | Option model | Correlation between interest rates | Median    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.554 0.440
Interest rate contracts | Option model | Correlation between interest rate and foreign exchange rate | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.154 0.057
Interest rate contracts | Option model | Correlation between interest rate and foreign exchange rate | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.510 0.600
Interest rate contracts | Option model | Correlation between interest rate and foreign exchange rate | Median    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.251 0.343
Interest rate contracts | Option model | Volatility | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.007 0.653
Interest rate contracts | Option model | Volatility | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.681 1.346
Interest rate contracts | Option model | Volatility | Median    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.568 0.735
Foreign exchange contracts | Option model    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Fair value ¥ 25,606 ¥ 25,319
Foreign exchange contracts | Option model | Recovery rate | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs   0.800
Foreign exchange contracts | Option model | Recovery rate | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs   0.900
Foreign exchange contracts | Option model | Recovery rate | Median    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs   0.850
Foreign exchange contracts | Option model | Correlation between interest rates | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.467 0.300
Foreign exchange contracts | Option model | Correlation between interest rates | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.572 0.700
Foreign exchange contracts | Option model | Correlation between interest rates | Median    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.519 0.456
Foreign exchange contracts | Option model | Correlation between interest rate and foreign exchange rate | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.192 0.192
Foreign exchange contracts | Option model | Correlation between interest rate and foreign exchange rate | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.510 0.600
Foreign exchange contracts | Option model | Correlation between interest rate and foreign exchange rate | Median    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.296 0.367
Foreign exchange contracts | Option model | Volatility | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs   0.107
Foreign exchange contracts | Option model | Volatility | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs   0.209
Foreign exchange contracts | Option model | Volatility | Median    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs   0.143
Foreign exchange contracts | Option model | Correlation between foreign exchange rates | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs   0.500
Foreign exchange contracts | Option model | Correlation between foreign exchange rates | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs   0.664
Foreign exchange contracts | Option model | Correlation between foreign exchange rates | Median    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs   0.582
Equity contracts | Option model    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Fair value ¥ 7,630 ¥ 3,524
Equity contracts | Option model | Volatility | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.201 0.200
Equity contracts | Option model | Volatility | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.420 0.355
Equity contracts | Option model | Volatility | Median    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.290 0.275
Equity contracts | Option model | Correlation between foreign exchange rate and equity    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.432  
Equity contracts | Option model | Correlation between foreign exchange rate and equity | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs   0.060
Equity contracts | Option model | Correlation between foreign exchange rate and equity | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs   0.500
Equity contracts | Option model | Correlation between foreign exchange rate and equity | Median    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.432 0.100
Equity contracts | Option model | Correlation between equities | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.120 0.057
Equity contracts | Option model | Correlation between equities | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.906 0.950
Equity contracts | Option model | Correlation between equities | Median    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading derivatives—net, Significant unobservable inputs 0.461 0.589
Corporate bonds | Discounted cash flow    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Fair value ¥ 107,198 ¥ 175,464
Corporate bonds | Discounted cash flow | Liquidity premium    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.000  
Corporate bonds | Discounted cash flow | Liquidity premium | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.000  
Corporate bonds | Discounted cash flow | Liquidity premium | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.002  
Corporate bonds | Discounted cash flow | Liquidity premium | Weighted average    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.000 0.001
Asset-backed securities | Internal model    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Fair value ¥ 1,940,331 ¥ 1,684,080
Asset-backed securities | Internal model | Asset correlations    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.020 0.020
Asset-backed securities | Internal model | Asset correlations | Weighted average    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.020 0.020
Asset-backed securities | Internal model | Discount factor | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.013 0.012
Asset-backed securities | Internal model | Discount factor | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.014 0.013
Asset-backed securities | Internal model | Discount factor | Weighted average    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.013 0.013
Asset-backed securities | Internal model | Prepayment rate    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.207 0.280
Asset-backed securities | Internal model | Prepayment rate | Weighted average    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.207 0.280
Asset-backed securities | Internal model | Probability of default | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.000 0.000
Asset-backed securities | Internal model | Probability of default | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.925 0.990
Asset-backed securities | Internal model | Probability of default | Weighted average    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0 0
Asset-backed securities | Internal model | Recovery rate    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.594 0.604
Asset-backed securities | Internal model | Recovery rate | Weighted average    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.594 0.604
Other debt securities | Discounted cash flow    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Fair value ¥ 667,093 ¥ 659,534
Other debt securities | Discounted cash flow | Liquidity premium | Minimum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.016 0.016
Other debt securities | Discounted cash flow | Liquidity premium | Maximum    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.032 0.032
Other debt securities | Discounted cash flow | Liquidity premium | Weighted average    
Fair Value Inputs, Assets and Liabilities, Quantitative Information [Line Items]    
Trading securities and Investment securities, Significant unobservable inputs 0.024 0.024