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Benefit Plans (Fair values of our pension plan assets by asset category) (Details) - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets $ 8,352 $ 7,765 $ 9,469
Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets 8,352 7,765  
Short-term investments [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets 218 98  
Short-term investments [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets 218 98  
Fixed income [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets 3,823 2,924  
Fixed income [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets 3,823 2,924  
Equity investments [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets 4,311 4,743  
Equity investments [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Fair values of the Corporation's pension plan assets $ 4,311 $ 4,743