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Summary of Significant Accounting Policies - Fair Value Assumptions for Stock Option Pricing (Details) - $ / shares
12 Months Ended
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Organization, Consolidation and Presentation of Financial Statements [Abstract]      
Number of stock options granted (in shares) 282,040 223,208 222,828
Weighted average grant date fair value of stock options granted (in usd per share) $ 6.26 $ 7.16 $ 6.57
Weighted average assumptions:      
Risk-free interest rate 2.50% 2.60% 2.00%
Expected dividend yield 2.90% 2.60% 2.50%
Expected life, in years 5 years 4 years 9 months 18 days 4 years 10 months 24 days
Expected volatility (based on historical volatility) 21.90% 21.50% 21.40%