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Retirement Plans, Information About Plan Assets and Asset Allocation (Details) - USD ($)
$ in Thousands
3 Months Ended 12 Months Ended
Mar. 31, 2018
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Higman [Member]        
Information about Plan Assets [Abstract]        
Decrease in pension plan projected benefit obligation   $ (3,081)    
Pension contributions $ 6,717      
Expected additional contribution   $ 1,385    
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)    
Pension contributions   8,102 $ 0  
Expected additional contribution   2,395 0  
Fair value of plan assets   $ 303,151 $ 294,995 $ 257,517
Expected long-term rate of return on plan assets   7.00% 7.00% 7.50%
Contribution percentage to defined contribution plan   85.00%    
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   52.00% 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   17.00% 21.00%  
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% 28.00%  
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   3.00% 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]        
Pension contributions   $ 103 $ 75  
Expected additional contribution   66 48  
Fair value of plan assets   $ 0 $ 0 $ 0
Pension plan defined benefit plan cost increase limit percentage   4.00%