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Stock Compensation Plans - Options Granted and Intrinsic Value (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Stock Compensation Plans      
Share Based Compensation Arrangement By Share Based Payment Award Options Granted Number 196,350 186,400 186,750
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercises in Period, Intrinsic Value $ 371,000 $ 3,100,000 $ 2,200,000
Proceeds from Stock Options Exercised 661,000 4,158,000 2,224,000
Share-based Payment Arrangement, Exercise of Option, Tax Benefit 257,000 2,700,000 1,600,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Outstanding, Intrinsic Value 4,500,000 11,500,000 4,700,000
Share-based Compensation Arrangement by Share-based Payment Award, Options, Exercisable, Intrinsic Value $ 2,900,000 $ 6,600,000 $ 3,900,000