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PRINCIPAL ACCOUNTING POLICIES - Earnings/(loss) per share, treasury stock and segment reporting (Details)
$ in Millions
12 Months Ended
Dec. 31, 2017
item
Dec. 31, 2015
Nov. 30, 2015
Apr. 30, 2014
USD ($)
Treasury stock disclosures        
Approved repurchase plan | $       $ 600
Segment reporting disclosures        
Number of reportable segments | item 1      
ADS        
Treasury stock disclosures        
Ratio of ADSs to ordinary shares   8 4