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STOCK-BASED COMPENSATION PLANS - Summary of assumptions used (Details) - Stock Appreciation Rights
9 Months Ended 12 Months Ended
Sep. 30, 2021
Dec. 31, 2020
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Risk-free interest rate 1.33% 0.80%
Expected volatility 35.36% 34.72%
Expected life (in years) 7 years 10 months 24 days 8 years 8 months 12 days
Expected dividend yield 3.86% 3.96%