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Summary of Significant Accounting Policies - Fair Value Assumptions for Stock Option Pricing (Details) - $ / shares
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]      
Number of stock options granted (in shares) 223,208 222,828 307,176
Weighted average grant date fair value of stock options granted (in usd per share) $ 7.16 $ 6.57 $ 5.01
Weighted average assumptions:      
Risk-free interest rate 2.60% 2.00% 1.30%
Expected dividend yield 2.60% 2.50% 3.20%
Expected life, in years 4 years 9 months 18 days 4 years 10 months 24 days 4 years 10 months 24 days
Expected volatility (based on historical volatility) 21.50% 21.40% 25.60%