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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, 2015
Dec. 31, 2014
Dec. 31, 2013
Net actuarial gain (loss), Gross $ (2,438) $ 54,136 $ (59,654)
Retirement Income Plan [Member]      
Net actuarial gain (loss), Gross (1,425) 52,556 (58,001)
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation, beginning of year 322,271 249,422  
Service cost 7,215 5,763 7,346
Interest cost 13,668 12,776 12,139
Actuarial losses (gains) (24,994) 61,534  
Benefits Paid Benefit Obligation (7,852) (7,224)  
Defined Benefit Plan, Curtailments 0 0  
Benefit obligation, end of year 310,308 322,271 249,422
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]      
Fair value of assets, beginning of year 253,452 225,817  
Actual return on plan assets, net of expenses (7,600) 24,649  
Contributions by the employer to funded plans 11,700 10,210  
Benefits paid fair value (7,852) (7,224)  
Fair value of assets, end of year 249,700 253,452 $ 225,817
Funded status (60,608) (68,819)  
Defined Benefit Plan, Amounts Recognized in Balance Sheet [Abstract]      
Liabilities (60,608) (68,819)  
Net pension liability, end of year (60,608) (68,819)  
Pension and Other Postretirement Benefit Plans, Accumulated Other Comprehensive Income (Loss), before Tax [Abstract]      
Net actuarial loss 80,828 89,085  
Total $ 80,828 $ 89,085  
Weighted-Average Liability Asumptions      
Discount rate 4.69% 4.29%  
Rate of compensation increase 4.00% 4.00%  
Accumulated benefit obligation $ 310,307 $ 318,018  
Retirement Life Plan [Member]      
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]      
Benefit obligation, beginning of year 6,400    
Benefit obligation, end of year $ 6,000 $ 6,400