Derivative instruments and hedging activities - Schedule of information about written credit derivatives and purchased credit protection (Detail) - JPY (¥) ¥ in Billions		6 Months Ended	12 Months Ended
		Sep. 30, 2025	Mar. 31, 2025
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	¥ (268)	¥ (349)
Maximum potential payout / Notional		123,923	28,375
Notional, Purchased credit protection		(107,227)	(15,635)
Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	(134)	(156)
Maximum potential payout / Notional		16,864	11,480
Notional, Purchased credit protection		(10,615)	(6,711)
Credit default swap indices [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	(144)	(221)
Maximum potential payout / Notional		105,686	15,488
Notional, Purchased credit protection		(95,869)	(8,097)
Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	10	28
Maximum potential payout / Notional		1,266	1,236
Notional, Purchased credit protection		(686)	(785)
Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	0	0
Maximum potential payout / Notional		107	171
Notional, Purchased credit protection		(57)	(42)
Less than 1 year [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,519	3,319
Less than 1 year [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,552	1,730
Less than 1 year [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,841	1,465
Less than 1 year [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		126	124
Less than 1 year [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
1 to 3 years [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		9,734	6,767
1 to 3 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,782	3,124
1 to 3 years [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		5,570	3,168
1 to 3 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		372	464
1 to 3 years [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		10	11
3 to 5 years [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		68,359	13,490
3 to 5 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		9,027	4,963
3 to 5 years [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		58,513	7,877
3 to 5 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		724	571
3 to 5 years [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		95	79
More than 5 years [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		42,311	4,799
More than 5 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,503	1,663
More than 5 years [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		39,762	2,978
More than 5 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		44	77
More than 5 years [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		¥ 2	¥ 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.