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SHARE INCENTIVE PLANS - Employee stock purchase plan activity (Details) - ESPP - USD ($)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
May 31, 2016
Share-based compensation        
Maximum employee base compensation contribution 15.00%      
ESPP purchase price percentage of market price 85.00%      
Shares reserved for issuance pursuant to purchase rights under the ESPP 6,585,712     4,000,000
Maximum value of ordinary shares issued per employee pursuant to purchase rights under the ESPP per calendar year $ 25,000      
Shares issued in period 568,876 491,968 364,746  
Weighted-average exercise price, options granted (in USD per share) $ 38.83 $ 35.50 $ 35.15