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PRINCIPAL ACCOUNTING POLICIES - Foreign currencies and land use rights (Details) - ¥ / $
12 Months Ended
Dec. 31, 2017
Dec. 29, 2017
RMB/USD exchange rate used in translation   0.1537
Minimum    
Useful life 3 years  
Maximum    
Useful life 10 years  
Land use rights | Minimum    
Useful life 40 years  
Land use rights | Maximum    
Useful life 50 years