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, 2022	Mar. 31, 2022
Credit Derivatives [Line Items]
Maximum potential payout / Notional		¥ 20,756	¥ 18,257
Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		8,812	7,708
Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		11,412	10,015
Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		468	419
Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		64	115
AAA [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		465	297
AAA [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		231	192
AAA [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		234	105
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		1,852	1,700
AA [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,654	1,485
AA [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		198	215
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		6,628	5,622
A [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,626	2,164
A [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,939	3,369
A [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		63	28
A [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional			61
BBB [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		8,424	7,322
BBB [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,577	2,057
BBB [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		5,623	5,012
BBB [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		186	226
BBB [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		38	27
BB [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,054	1,931
BB [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		936	869
BB [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,028	988
BB [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		64	47
BB [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		26	27
Other [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	1,333	1,385
Other [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	788	941
Other [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	390	326
Other [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	155	118
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.