v3.26.1
Derivative instruments and hedging activities - Schedule of information about written credit derivatives and purchased credit protection (Detail) - JPY (¥)
¥ in Billions
 12 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Credit Derivatives [Line Items]    
Carrying value (Asset) / Liability [1] ¥ (944) ¥ (349)
Maximum potential payout / Notional 68,210 28,375
Notional, Purchased credit protection (52,484) (15,635)
Single-name credit default swaps [Member]    
Credit Derivatives [Line Items]    
Carrying value (Asset) / Liability [1] (277) (156)
Maximum potential payout / Notional 23,468 11,480
Notional, Purchased credit protection (17,962) (6,711)
Credit default swap indices [Member]    
Credit Derivatives [Line Items]    
Carrying value (Asset) / Liability [1] (617) (221)
Maximum potential payout / Notional 42,499 15,488
Notional, Purchased credit protection (33,274) (8,097)
Other credit risk related portfolio products [Member]    
Credit Derivatives [Line Items]    
Carrying value (Asset) / Liability [1] (49) 28
Maximum potential payout / Notional 1,824 1,236
Notional, Purchased credit protection (1,029) (785)
Credit-risk related options and swaptions [Member]    
Credit Derivatives [Line Items]    
Carrying value (Asset) / Liability [1] (1) 0
Maximum potential payout / Notional 419 171
Notional, Purchased credit protection (219) (42)
Less than 1 year [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 5,271 3,319
Less than 1 year [Member] | Single-name credit default swaps [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 2,229 1,730
Less than 1 year [Member] | Credit default swap indices [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 2,859 1,465
Less than 1 year [Member] | Other credit risk related portfolio products [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 177 124
Less than 1 year [Member] | Credit-risk related options and swaptions [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 6
1 to 3 years [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 16,679 6,767
1 to 3 years [Member] | Single-name credit default swaps [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 6,782 3,124
1 to 3 years [Member] | Credit default swap indices [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 9,014 3,168
1 to 3 years [Member] | Other credit risk related portfolio products [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 881 464
1 to 3 years [Member] | Credit-risk related options and swaptions [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 2 11
3 to 5 years [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 38,281 13,490
3 to 5 years [Member] | Single-name credit default swaps [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 11,699 4,963
3 to 5 years [Member] | Credit default swap indices [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 25,640 7,877
3 to 5 years [Member] | Other credit risk related portfolio products [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 568 571
3 to 5 years [Member] | Credit-risk related options and swaptions [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 374 79
More than 5 years [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 7,979 4,799
More than 5 years [Member] | Single-name credit default swaps [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 2,758 1,663
More than 5 years [Member] | Credit default swap indices [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 4,986 2,978
More than 5 years [Member] | Other credit risk related portfolio products [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional 198 77
More than 5 years [Member] | Credit-risk related options and swaptions [Member]    
Credit Derivatives [Line Items]    
Maximum potential payout / Notional ¥ 37 ¥ 81
[1] Carrying value amounts are shown on a gross basis prior to cash collateral or counterparty offsetting. Asset balances represent positive fair value amounts caused by tightening of credit spreads of the underlyings since inception of the credit derivatives.