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, 2018	Mar. 31, 2018
Credit Derivatives [Line Items]
Maximum potential payout / Notional		¥ 14,164	¥ 14,419
Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		8,740	9,271
Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		5,180	4,842
Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		244	306
AAA [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		516	617
AAA [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		494	466
AAA [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		22	135
AAA [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional			16
AA [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		936	972
AA [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		903	928
AA [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		33	44
AA [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional
A [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		4,109	4,546
A [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,411	2,763
A [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,697	1,779
A [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1	4
BBB [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		5,580	5,493
BBB [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		3,283	3,476
BBB [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,237	1,949
BBB [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		60	68
BB [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		2,414	2,223
BB [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		1,341	1,369
BB [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		945	736
BB [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional		128	118
Other [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	609	568
Other [Member] | Single-name credit default swaps [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	308	269
Other [Member] | Credit default indices [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	246	199
Other [Member] | Other credit risk related portfolio products [Member]
Credit Derivatives [Line Items]
Maximum potential payout / Notional	[1]	¥ 55	¥ 100

[1] "Other" includes credit derivatives where the credit rating of the underlying reference asset is below investment grade or where a rating is unavailable.