SUMMARY OF PRINCIPAL ACCOUNTING POLICIES - Long-term Investment (Details) ¥ in Thousands, $ in Thousands	12 Months Ended
Dec. 31, 2020 USD ($)	Dec. 31, 2020 CNY (¥)	Dec. 31, 2019 CNY (¥)	Dec. 31, 2018 CNY (¥)
Long-term Investments
Impairment of long-term investments	$ 219	¥ 1,430	¥ 1,320	¥ 0

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=121553693&loc=SL6284422-111562

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