SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Fulfilment costs (Details) - CNY (¥) ¥ in Thousands	Dec. 31, 2021	Dec. 31, 2020
Other non-current assets
Capitalized fulfilment cost	¥ 80,742	¥ 128,983

Amount, after accumulated amortization and accumulated impairment loss, of asset recognized from cost incurred to obtain or fulfill contract with customer; classified as noncurrent.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 340
-SubTopic 40
-Section 50
-Paragraph 3
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=123353750&loc=SL49131252-203054

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