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SHAREHOLDERS' EQUITY - Weighted-Average Assumptions Used to Estimate Fair Value of Options (Details) - Employee Stock Option
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected annual dividend yield 2.40% 0.00% 0.00%
Risk-free interest rate 2.26% 1.64% 1.73%
Expected volatility 48.90% 44.40% 40.20%
Expected life in years 4 years 7 years 1 month 6 days 6 years 3 months 18 days