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Stock Plans - Weighted-Average Black-Scholes Fair Value Assumptions (Details) - $ / shares
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Share-based Payment Arrangement [Abstract]      
Risk-Free Interest Rate 1.34% 2.46% 2.68%
Expected Life 5 years 5 years 5 years
Expected Volatility 19.64% 19.96% 20.96%
Expected Dividend Yield 2.06% 2.37% 2.02%
Fair Value at Grant Date (in dollars per share) $ 8.61 $ 7.83 $ 8.30