SUMMARY OF PRINCIPAL ACCOUNTING POLICIES - Recently issued accounting pronouncements (Details) $ in Millions	12 Months Ended
Dec. 31, 2017 USD ($)
SUMMARY OF PRINCIPAL ACCOUNTING POLICIES
Cumulative effective adjustment, Increase in accumulated earnings	$ 1.3

Amount of the cumulative effect on retained earnings net of related income tax effect.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 250
-SubTopic 10
-Section 50
-Paragraph 9
-URI http://asc.fasb.org/extlink&oid=109234566&loc=d3e22663-107794

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration