Securitizations and Variable Interest Entities - Fair value of retained interests, key economic assumptions and sensitivity (Parenthetical) (Detail) - JPY (¥) ¥ in Billions	6 Months Ended	12 Months Ended
Sep. 30, 2017	Mar. 31, 2017
Variable Interest Entity [Line Items]
Sensitivity analysis covers the material retained interests held	¥ 175	¥ 285
Retained interests	¥ 200	¥ 308
Minimum [Member]
Variable Interest Entity [Line Items]
Percentage of changes in fair value based on adverse effect	10.00%	10.00%
Maximum [Member]
Variable Interest Entity [Line Items]
Percentage of changes in fair value based on adverse effect	20.00%	20.00%

The carrying amount (fair value) as of the balance sheet date of the transferor's interests in transferred financial assets (typically securitized receivables, generally not involved in the collection of principal) following transfer of significant portions of such financial instruments to other parties. Such interests could be ancillary cash flows and interest-only strips.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=82924980&loc=d3e107314-111719

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