PRINCIPAL ACCOUNTING POLICIES - Sales and marketing expenses and Business combinations and non-controlling interests (Details) - CNY (¥) ¥ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2020	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2019
PRINCIPAL ACCOUNTING POLICIES
Advertising costs	¥ 50.0	¥ 58.7	¥ 128.9	¥ 443.6

Amount charged to advertising expense for the period, which are expenses incurred with the objective of increasing revenue for a specified brand, product or product line.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 720
-SubTopic 35
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=6420018&loc=d3e36677-107848

Name:	us-gaap_AdvertisingExpense
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration