Summary of significant accounting policies (Finance Receivables 90 Days or More Past Due and Accruing) (Detail) - JPY (¥) ¥ in Millions	Mar. 31, 2020	Mar. 31, 2019
Financing Receivable, Recorded Investment, Past Due [Line Items]
Finance receivables past due over 90 days and still accruing	¥ 42,870	¥ 32,259
Retail Receivables Portfolio Segment
Financing Receivable, Recorded Investment, Past Due [Line Items]
Finance receivables past due over 90 days and still accruing	37,709	28,438
Finance Lease Receivables Portfolio Segment
Financing Receivable, Recorded Investment, Past Due [Line Items]
Finance receivables past due over 90 days and still accruing	¥ 5,161	¥ 3,821

Amount of financing receivable 90 days or more past due and still accruing.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 16
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919258-210447

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 7
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=d3e5111-111524

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 80
-URI http://asc.fasb.org/extlink&oid=118955255&loc=SL82922355-210448

Name:	us-gaap_FinancingReceivableRecordedInvestment90DaysPastDueAndStillAccruing
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant