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Retirement Plans, Information About Plan Assets and Asset Allocation (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Real Estate Investment [Member] | Level 3 [Member]      
Information about Plan Assets [Abstract]      
Fair value of plan assets $ 138,000 $ 125,000  
Cash and Equity Securities [Member] | Level 1 [Member]      
Information about Plan Assets [Abstract]      
Fair value of plan assets 25,032,000 25,871,000  
Higman [Member]      
Information about Plan Assets [Abstract]      
Decrease in pension plan projected benefit obligation (3,081,000)    
Plan 2018 [Member] | Higman [Member]      
Information about Plan Assets [Abstract]      
Pension contributions   1,615,000 $ 1,385,000
Plan 2019 [Member] | Higman [Member]      
Information about Plan Assets [Abstract]      
Pension contributions 797,000 1,449,000  
Plan 2020 [Member] | Higman [Member]      
Information about Plan Assets [Abstract]      
Pension contributions $ 1,438,000    
Plan 2016 and 2017 [Member] | Higman [Member]      
Information about Plan Assets [Abstract]      
Pension contributions     6,717,000
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    
Pension contributions $ 2,235,000 3,064,000  
Fair value of plan assets $ 395,137,000 $ 358,197,000 $ 303,151,000
Expected long-term rate of return on plan assets 6.75% 7.00% 7.00%
Contribution percentage to defined contribution plan 82.00%    
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 53.00% 53.00%  
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 20.00% 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 25.00% 26.00%  
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 2.00% 2.00%  
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 $ 139,000 $ 90,000  
Fair value of plan assets $ 0 $ 0 $ 0
Pension plan defined benefit plan cost increase limit percentage 4.00%