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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, 2017
Dec. 31, 2016
Dec. 31, 2015
Deferred Compensation Arrangement with Individual, Contributions by Employer $ 200 $ 300 $ 200
Net actuarial gain (loss), Gross 4,684 12,079 (2,438)
Supplemental Employee Retirement Plan [Member]      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation, beginning of year 9,100    
Benefit obligation, end of year 10,100 9,100  
Retirement Income Plan [Member]      
Defined Benefit Plan, Amortization of Gain (Loss) (2,001) (6,299) (6,831)
Net actuarial gain (loss), Gross 3,594 11,316 (1,425)
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation, beginning of year 330,588 310,308  
Service cost 0 1,647 7,215
Interest cost 12,490 12,336 13,668
Actuarial losses (gains) 31,158 15,086  
Benefit obligation, end of year 364,411 330,588 310,308
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of assets, beginning of year 316,515 249,700  
Actual return on plan assets, net of expenses 46,983 21,079  
Contributions by the employer to funded plans 10,000 54,525  
Benefits paid fair value (9,825) (8,789)  
Fair value of assets, end of year 363,673 316,515 $ 249,700
Funded status (738) (14,073)  
Defined Benefit Plan, Amounts for Asset (Liability) Recognized in Statement of Financial Position [Abstract]      
Liabilities (738) (14,073)  
Net pension liability, end of year (738) (14,073)  
Defined Benefit Plan, Accumulated Other Comprehensive (Income) Loss, before Tax [Abstract]      
Net actuarial loss 87,438 85,845  
Total 87,438 85,845  
Weighted-Average Liability Asumptions      
Accumulated benefit obligation $ 364,411 $ 330,588  
Discount rate 3.78% 4.41%  
Retirement Life Plan [Member]      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation, beginning of year $ 6,300    
Benefit obligation, end of year $ 6,400 $ 6,300