SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Employee benefit expenses and Segment reporting (Details) ¥ in Thousands, $ in Thousands	12 Months Ended
Dec. 31, 2021 CNY (¥) segment	Dec. 31, 2021 USD ($) segment	Dec. 31, 2020 CNY (¥)	Dec. 31, 2019 CNY (¥)
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Employee benefit expenses	¥ 247,315	$ 38,809	¥ 139,621	¥ 197,327
Number of reportable segments	1	1

Number of segments reported by the entity. A reportable segment is a component of an entity for which there is an accounting requirement to report separate financial information on that component in the entity's financial statements.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 18
-URI http://asc.fasb.org/extlink&oid=123359005&loc=d3e8672-108599

Name:	us-gaap_NumberOfReportableSegments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration