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Retirement Plans (Funded Status of Retirement Income Plan and Retirement Life Plan) (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Retirement Income Plan [Member]      
Defined Benefit Plan, Assumptions Used Calculating Benefit Obligation, Discount Rate 2.68% 3.33%  
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [1] $ (7,778) $ (5,033) $ (8,358)
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation, beginning of year 391,021 334,679  
Interest cost 11,312 13,506 12,428
Actuarial losses (gains) 35,276 54,478  
Benefits paid fair value (12,448) (11,642)  
Benefit obligation, end of year 425,161 391,021 334,679
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of assets, beginning of year 385,087 331,680  
Actual return on plan assets, net of expenses 60,077 63,949  
Contributions by the employer to funded plans 0 1,100  
Fair value of assets, end of year 432,716 385,087 331,680
Funded status 7,555 (5,934)  
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]      
Assets 7,555 0  
Liabilities 0 5,934  
Net pension asset (liability), end of year 7,555 (5,934)  
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]      
Net actuarial loss 101,414 107,125  
Weighted-Average Liability Asumptions      
Accumulated benefit obligation 425,161 391,021  
Retirement Life Plan [Member]      
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Contributions by the employer to funded plans $ 18,600 $ 17,300 $ 16,200
[1] The components of net periodic pension cost (benefit) are included within "Loss and loss expense incurred" and "Other insurance expenses" on the Consolidated Statements of Income.