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Postretirement Benefit Plans (Plan Assets at Fair Value) (Details) - Defined Benefit Pension Plan [Member] - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Dec. 31, 2017
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan fair value of plan assets $ 5,404 $ 4,818 $ 8,121
Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan fair value of plan assets 4,701 4,268  
Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan fair value of plan assets 703 550  
Cash and Cash Equivalents [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan fair value of plan assets 2,797 2,430  
Mining [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan fair value of plan assets 34 36  
Manufacturing [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan fair value of plan assets 830 888  
Transportation, Communications, Electric, Gas, and Sanitary Services [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan fair value of plan assets 543 468  
Finance, Insurance, and Real Estate [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan fair value of plan assets 330 248  
Services [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan fair value of plan assets 155 189  
Other [Member] | Level 1 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan fair value of plan assets 12 9  
Corporate debt securities [Member] | Level 2 [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Defined benefit plan fair value of plan assets $ 703 $ 550