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PRINCIPAL ACCOUNTING POLICIES - Summary of share option activity (Details) - USD ($)
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Dec. 31, 2014
Number of Shares        
Outstanding, beginning 5,532,515 5,005,461 4,972,178  
Granted 631,166 937,002 625,006  
Exercised (1,353,697) (368,722) (506,163)  
Forfeited (99,226) (41,226) (85,560)  
Outstanding, ending 4,710,758 5,532,515 5,005,461 4,972,178
Vested and expect to vest, ending 4,570,018      
Exercisable, ending 2,951,517      
Weighted Average Exercise Price        
Outstanding, beginning (in dollars per share) $ 157.82 $ 122.00 $ 101.03  
Granted (in dollars per share) 43.03 322.20 247.91  
Exercised (in dollars per share) 68.95 80.70 65.65  
Forfeited (in dollars per share) 133.92 199.32 156.32  
Outstanding, ending (in dollars per share) 168.80 $ 157.82 $ 122.00 $ 101.03
Vested and expect to vest, ending (in dollars per share) 167.40      
Exercisable, ending (in dollars per share) $ 141.77      
Weighted Average Remaining Contractual Life (Years)        
Outstanding 4 years 8 months 19 days 4 years 6 months 15 days 4 years 9 months 7 days 5 years 2 months 1 day
Vested and expect to vest at the end of the period 4 years 8 months 1 day      
Exercisable at the end of the period 3 years 9 months 11 days      
Aggregate Intrinsic Value        
Outstanding $ 866,781,417 $ 899,966,907 $ 1,244,544,670 $ 405,399,251
Vested and expect to vest 847,267,373      
Exercisable $ 622,855,872      
Assumptions        
Pre-vesting forfeiture rate assumptions (as a percent) 8.00%      
Closing stock price (in dollars per share) $ 352.80      
Total intrinsic value of options exercised $ 522,000,000 $ 257,000,000 $ 178,000,000