Derivative instruments and hedging activities - Schedule of information about written credit derivatives by external credit rating of underlying asset (Detail) - JPY (¥) ¥ in Billions		Mar. 31, 2017	Mar. 31, 2016
Credit Derivatives [Line Items]
Maximum potential payout / Notional		¥ 17,604	¥ 21,828
Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		12,029	15,609
Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		5,130	5,797
Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		445	355
Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional			67
AAA Rating [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,033	1,427
AAA Rating [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		843	1,230
AAA Rating [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		171	178
AAA Rating [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		19	19
AAA Rating [Member] | Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
AA Rating [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,213	1,320
AA Rating [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,186	1,305
AA Rating [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		27	15
AA Rating [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
AA Rating [Member] | Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
A Rating [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		6,943	8,657
A Rating [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,658	4,407
A Rating [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,284	4,249
A Rating [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1	1
A Rating [Member] | Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
BBB Rating [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		5,231	6,437
BBB Rating [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		4,211	5,428
BBB Rating [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,017	939
BBB Rating [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3	3
BBB Rating [Member] | Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional			67
BB Rating [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,079	2,468
BB Rating [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,486	2,243
BB Rating [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		474	224
BB Rating [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		119	1
BB Rating [Member] | Credit risk related options and swaptions [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
Other Rating [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	1,105	1,519
Other Rating [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	645	996
Other Rating [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	157	192
Other Rating [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	303	331
Other Rating [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 rating is unavailable.