Derivative instruments and hedging activities - Concentration of Exposures to Credit Risk in OTC Derivatives (Detail) - JPY (¥) ¥ in Billions	Mar. 31, 2015	Mar. 31, 2014
Derivative [Line Items]
Gross fair value of derivative assets, Financial institutions	¥ 42,100	¥ 26,069
OTC derivatives with financial institutions [Member]
Derivative [Line Items]
Gross fair value of derivative assets, Financial institutions	33,930	20,355
Impact of master netting agreements, Financial institutions	(31,773)	(18,481)
Impact of collateral, Financial institutions	(1,713)	(936)
Net exposure to credit risk, Financial institutions	¥ 444	¥ 938

Fair value, before effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets elected not to be offset. Excludes assets not subject to a master netting arrangement.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 815

-SubTopic 10

-Section 50

-Paragraph 4B

-Subparagraph (a),(c)

-URI http://asc.fasb.org/extlink&oid=35706170&loc=SL5624163-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 50

-Paragraph 3

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312

Reference 3: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 20

-Section 55

-Paragraph 7

-URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226000-175313

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant