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, 2017	Mar. 31, 2017
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	¥ (75)	¥ (38)
Maximum potential payout / Notional		15,238	17,604
Notional Purchased credit protection		10,479	13,113
Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	(22)	(17)
Maximum potential payout / Notional		10,032	12,029
Notional Purchased credit protection		7,582	9,536
Credit default indices [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	(57)	(26)
Maximum potential payout / Notional		4,732	5,130
Notional Purchased credit protection		2,568	3,265
Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	4	5
Maximum potential payout / Notional		471	445
Notional Purchased credit protection		326	312
Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	0
Maximum potential payout / Notional		3
Notional Purchased credit protection		3
Less than 1 year [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,597	3,771
Less than 1 year [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,487	2,908
Less than 1 year [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		853	697
Less than 1 year [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		257	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		5,284	6,308
1 to 3 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,942	4,497
1 to 3 years [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,166	1,558
1 to 3 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		176	253
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		4,551	5,621
3 to 5 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,403	3,414
3 to 5 years [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,119	2,188
3 to 5 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		29	19
3 to 5 years [Member] | Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
More than 5 years [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,806	1,904
More than 5 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,200	1,210
More than 5 years [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		594	687
More than 5 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		9	7
More than 5 years [Member] | Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		¥ 3

[1] Carrying value amounts are shown on a gross basis prior to cash collateral or counterparty netting. Asset balances represent positive fair value amounts caused by tightening of credit spreads of underlyings since inception of the credit derivative contracts.