Derivative instruments and hedging activities - Net Investment Hedges (Detail) - Net investment hedges [Member] - JPY (¥) ¥ in Billions	12 Months Ended
Mar. 31, 2016	Mar. 31, 2015	Mar. 31, 2014
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivatives and non-derivatives designated as net investment hedges	¥ 7	¥ 7	¥ (12)
Foreign Exchange Contract [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Gains (losses) from derivatives and non-derivatives designated as net investment hedges	¥ 7	¥ 7	¥ (12)

The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

Name:	us-gaap_DerivativeInstrumentsGainLossRecognizedInOtherComprehensiveIncomeEffectivePortionNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration