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STOCK-BASED COMPENSATION - Summary of Assumptions Used in Total Stockholder Return-Based Stock Units Valuation (Details) - TSR Units
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Assumptions used in fair value valuation      
Expected dividend yield (assuming full reinvestment) 0.00% 0.00% 0.00%
Minimum      
Assumptions used in fair value valuation      
Risk free interest rate 1.42% 0.78% 0.87%
Expected stock price volatility 28.86% 27.40% 23.55%
Expected service period 2 years 2 months 12 days 2 years 3 months 18 days 2 years 6 months
Maximum      
Assumptions used in fair value valuation      
Risk free interest rate 1.99% 1.60% 1.31%
Expected stock price volatility 30.97% 28.42% 27.02%
Expected service period 3 years 3 years 3 years