Derivative instruments and hedging activities - Schedule of information about written credit derivatives and purchased credit protection (Detail) - JPY (¥) ¥ in Billions		12 Months Ended
		Mar. 31, 2017	Mar. 31, 2016
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	¥ (38)	¥ 195
Maximum potential payout / Notional		17,604	21,828
Notional Purchased credit protection		13,113	17,367
Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	(17)	131
Maximum potential payout / Notional		12,029	15,609
Notional Purchased credit protection		9,536	12,796
Credit default indices [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	(26)	52
Maximum potential payout / Notional		5,130	5,797
Notional Purchased credit protection		3,265	4,295
Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]	5	12
Maximum potential payout / Notional		445	355
Notional Purchased credit protection		312	209
Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Carrying value (Asset) / Liability	[1]		0
Maximum potential payout / Notional			67
Notional Purchased credit protection			67
Less than 1 year [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,771	4,647
Less than 1 year [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,908	3,658
Less than 1 year [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		697	918
Less than 1 year [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		166	71
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		6,308	7,163
1 to 3 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		4,497	5,292
1 to 3 years [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,558	1,623
1 to 3 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		253	248
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		5,621	7,848
3 to 5 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,414	5,252
3 to 5 years [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,188	2,505
3 to 5 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		19	24
3 to 5 years [Member] | Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional			67
More than 5 years [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,904	2,170
More than 5 years [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,210	1,407
More than 5 years [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		687	751
More than 5 years [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		7	12
More than 5 years [Member] | Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional

[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.