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, 2022	Mar. 31, 2022
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	¥ 152	¥ (102)
Maximum potential payout / Notional		20,756	18,257
Notional, Purchased credit protection		15,262	13,543
Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	59	19
Maximum potential payout / Notional		8,812	7,708
Notional, Purchased credit protection		6,474	5,688
Credit default swap indices [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	21	(140)
Maximum potential payout / Notional		11,412	10,015
Notional, Purchased credit protection		8,476	7,494
Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	71	19
Maximum potential payout / Notional		468	419
Notional, Purchased credit protection		285	293
Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	1	0
Maximum potential payout / Notional		64	115
Notional, Purchased credit protection		27	68
Less than 1 year [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		4,347	3,440
Less than 1 year [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,567	1,339
Less than 1 year [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,691	2,045
Less than 1 year [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		89	56
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		8,337	7,390
1 to 3 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,409	2,915
1 to 3 years [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		4,689	4,189
1 to 3 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		239	286
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		6,014	5,856
3 to 5 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,629	2,448
3 to 5 years [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,203	3,257
3 to 5 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		118	63
3 to 5 years [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		64	88
More than 5 years [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,058	1,571
More than 5 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,207	1,006
More than 5 years [Member] | Credit default swap indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		829	524
More than 5 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		22	14
More than 5 years [Member] | Credit-risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional			¥ 27

[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.