PRINCIPAL ACCOUNTING POLICIES - Warranty service revenue 1 (Details) - Revenue, Remaining Performance Obligation, Expected Timing of Satisfaction, Start Date [Axis]: 2020-04-01	Mar. 31, 2022
Product Warranty Liability [Line Items]
Performance obligation period	12 months
Minimum
Product Warranty Liability [Line Items]
Performance obligation period	6 years
Maximum
Product Warranty Liability [Line Items]
Performance obligation period	1 year

Period in which remaining performance obligation is expected to be recognized as revenue, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 13
-Subparagraph (b)(1)
-URI https://asc.fasb.org/extlink&oid=126920106&loc=SL49130556-203045

Name:	us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant