Derivative instruments and hedging activities - Additional information (Detail) - JPY (¥) ¥ in Billions	Mar. 31, 2019	Mar. 31, 2018
Derivative Instruments and Hedging Activities [Abstract]
Derivative liability position with credit-risk-related contingent features	¥ 486	¥ 406
Collateral pledged for derivative instruments with credit-risk-related contingent features that are in a liability position	410	314
Additional collateral required to be posted, aggregate fair value	¥ 3	¥ 3

The aggregate fair value of additional assets that would be required to be posted as collateral for derivative instruments with credit-risk-related contingent features if the credit-risk-related contingent features were triggered at the end of the reporting period.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4H
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5624258-113959

Name:	us-gaap_AdditionalCollateralAggregateFairValue
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant