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, 2023	Mar. 31, 2023
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	¥ (78)	¥ (38)
Maximum potential payout / Notional		17,135	15,635
Notional, Purchased credit protection		11,506	9,986
Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	(67)	(29)
Maximum potential payout / Notional		8,743	8,121
Notional, Purchased credit protection		6,496	5,708
Credit default swap indices [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	(52)	(47)
Maximum potential payout / Notional		7,700	6,839
Notional, Purchased credit protection		4,549	3,886
Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	41	38
Maximum potential payout / Notional		688	624
Notional, Purchased credit protection		457	341
Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	0	0
Maximum potential payout / Notional		4	51
Notional, Purchased credit protection		4	51
Less than 1 year [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,731	2,768
Less than 1 year [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,546	1,263
Less than 1 year [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,036	1,339
Less than 1 year [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		149	166
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		5,719	5,912
1 to 3 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,087	3,095
1 to 3 years [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,413	2,601
1 to 3 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		219	216
1 to 3 years [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
3 to 5 years [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		5,485	5,110
3 to 5 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,841	2,579
3 to 5 years [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,380	2,284
3 to 5 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		260	210
3 to 5 years [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		4	37
More than 5 years [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,200	1,845
More than 5 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,269	1,184
More than 5 years [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		871	615
More than 5 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		60	32
More than 5 years [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional			¥ 14

[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 underlyings since inception of the credit derivatives.