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Retirement Plans, Information About Plan Assets and Asset Allocation (Details) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension Plan [Member]      
Information about Plan Assets [Abstract]      
Service period of participants not impacted by pension plan amendment 15 years    
Age of plan participants not impacted by pension plan amendment 50 years    
Service period of participants who are age fifty not impacted by pension plan amendment 10 years    
Decrease in pension plan projected benefit obligation $ (33,433)    
Fair value of plan assets $ 294,995 $ 257,517 $ 243,588
Expected long-term rate of return on plan assets 7.00% 7.00% 7.50%
Contribution percentage to defined contribution plan 91.00%    
Asset Allocation Among Asset Categories [Abstract]      
Asset allocations 100.00% 100.00%  
Pension Plan [Member] | U.S. Equity Securities [Member]      
Asset Allocation Among Asset Categories [Abstract]      
Asset allocations 51.00% 51.00%  
Target allocations 50.00%    
Pension Plan [Member] | U.S. Equity Securities [Member] | Minimum [Member]      
Asset Allocation Among Asset Categories [Abstract]      
Target allocations 30.00%    
Pension Plan [Member] | U.S. Equity Securities [Member] | Maximum [Member]      
Asset Allocation Among Asset Categories [Abstract]      
Target allocations 70.00%    
Pension Plan [Member] | International Equity Securities [Member]      
Asset Allocation Among Asset Categories [Abstract]      
Asset allocations 21.00% 19.00%  
Target allocations 20.00%    
Pension Plan [Member] | International Equity Securities [Member] | Minimum [Member]      
Asset Allocation Among Asset Categories [Abstract]      
Target allocations 0.00%    
Pension Plan [Member] | International Equity Securities [Member] | Maximum [Member]      
Asset Allocation Among Asset Categories [Abstract]      
Target allocations 30.00%    
Pension Plan [Member] | Debt Securities [Member]      
Asset Allocation Among Asset Categories [Abstract]      
Asset allocations 28.00% 30.00%  
Target allocations 30.00%    
Pension Plan [Member] | Debt Securities [Member] | Minimum [Member]      
Asset Allocation Among Asset Categories [Abstract]      
Target allocations 15.00%    
Pension Plan [Member] | Debt Securities [Member] | Maximum [Member]      
Asset Allocation Among Asset Categories [Abstract]      
Target allocations 55.00%    
Pension Plan [Member] | Cash and Cash Equivalents [Member]      
Asset Allocation Among Asset Categories [Abstract]      
Asset allocations 0.00% 0.00%  
Target allocations 0.00%    
Pension Plan [Member] | Cash and Cash Equivalents [Member] | Minimum [Member]      
Asset Allocation Among Asset Categories [Abstract]      
Target allocations 0.00%    
Pension Plan [Member] | Cash and Cash Equivalents [Member] | Maximum [Member]      
Asset Allocation Among Asset Categories [Abstract]      
Target allocations 5.00%    
Other Postretirement Benefits [Member]      
Information about Plan Assets [Abstract]      
Fair value of plan assets $ 0 $ 0 $ 0
Pension plan defined benefit plan cost increase limit percentage 4.00%