TAXATION - Movement of valuation allowance (Details) - CNY (¥) ¥ in Thousands	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
TAXATION
Balance at beginning of the year	¥ 1,841,225	¥ 1,404,254	¥ 934,327
Valuation Allowance, Deferred Tax Asset, Increase (Decrease), Amount	354,645	436,971	469,927
Balance at end of the year	¥ 2,195,870	¥ 1,841,225	¥ 1,404,254

Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319

Name:	us-gaap_DeferredTaxAssetsValuationAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant