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Equity Incentive Plan - Valuation assumption (Details) - Performance Units And Shares [Member]
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Expected term (years) 2 years 10 months 24 days 2 years 10 months 24 days 2 years 10 months 24 days
Volatility 18.40% 19.70% 19.10%
Dividend yield 2.90% 3.40% 4.40%
Risk-free rate 1.30% 2.50% 2.40%