PARENT COMPANY ONLY CONDENSED FINANCIAL INFORMATION - Condensed Statement of Comprehensive Loss (Parenthetical) (Details) - CNY (¥) ¥ in Thousands	12 Months Ended	36 Months Ended
Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2019
Condensed statements of comprehensive loss
Foreign currency translation adjustments, tax	¥ 0	¥ 0	¥ 0
Parent Company
Condensed statements of comprehensive loss
Other comprehensive (loss) income, tax	0	0	0	¥ 0
Foreign currency translation adjustments, tax	¥ 0	¥ 0	¥ 0

Amount of tax expense (benefit), after reclassification adjustments of gain (loss) on foreign currency translation adjustments, foreign currency transactions designated and effective as economic hedges of a net investment in a foreign entity and intra-entity foreign currency transactions that are of a long-term-investment nature, attributable to parent entity.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
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