Derivative instruments and hedging activities - Schedule of information about written credit derivatives by external credit rating of underlying asset (Detail) - JPY (¥) ¥ in Billions		Sep. 30, 2023	Mar. 31, 2023
Credit Derivatives [Line Items]
Maximum potential payout / Notional		¥ 17,135	¥ 15,635
Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		8,743	8,121
Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		7,700	6,839
Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		688	624
Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		4	51
AAA [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		507	412
AAA [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		254	227
AAA [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		253	185
AAA [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
AAA [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
AA [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,543	1,585
AA [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,330	1,405
AA [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		213	180
AA [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
AA [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
A [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		5,697	5,323
A [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,667	2,378
A [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,008	2,924
A [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		22	21
A [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
BBB [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		6,640	5,728
BBB [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,894	2,530
BBB [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,413	2,844
BBB [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		333	325
BBB [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional			29
BB [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,182	1,155
BB [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		861	781
BB [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		291	299
BB [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		26	53
BB [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		4	22
Other [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	1,566	1,432
Other [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	737	800
Other [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	522	407
Other [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	307	225
Other [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]

[1] Other includes credit derivatives where the credit rating of the underlying reference asset is below investment grade or where a credit rating is unavailable.