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Share-Based Compensation - Assumptions Used to Estimate Fair Value of TSR grants (Details) - TSR Performance Share Awards
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Weighted-average volatility 35.00% 34.00% 33.00%
Expected dividend yield 2.60% 2.70% 2.10%
Expected term (in years) 3 years 3 years 3 years
Weighted average risk free interest rate 1.40% 2.40% 2.30%