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Benefit Plans (Fair values of our pension plan assets by asset category) (Details) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets $ 8,263 $ 8,352 $ 7,765
Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets 8,263 8,352  
Short-term investments [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets 69 218  
Short-term investments [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets 69 218  
Fixed income [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets 3,851 3,823  
Fixed income [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets 3,851 3,823  
Equity investments [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets 4,343 4,311  
Equity investments [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets $ 4,343 $ 4,311