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Retirement Plans (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Retirement Plans [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 plan defined benefit plan cost increase limit percentage     4.00%  
Higman [Member]        
Retirement Plans [Abstract]        
Pension contributions     $ 479,000  
Pension Plans [Member]        
Components of net periodic benefit cost [Abstract]        
Service cost $ 2,053 $ 1,919 3,980 $ 3,836
Interest cost 3,535 3,949 7,119 7,839
Expected return on plan assets (6,559) (5,735) (13,133) (11,923)
Amortization of actuarial loss (gain) 790 425 1,888 657
Net periodic benefit cost (181) 558 (146) 409
SERP [Member]        
Components of net periodic benefit cost [Abstract]        
Service cost 0 0 0 0
Interest cost 7 10 15 20
Expected return on plan assets 0 0 0 0
Amortization of actuarial loss (gain) 10 9 20 18
Net periodic benefit cost 17 19 35 38
Other Postretirement Benefits [Member]        
Components of net periodic benefit cost [Abstract]        
Service cost 0 0 0 0
Interest cost 4 5 8 11
Amortization of actuarial loss (gain) (112) (130) (225) (261)
Net periodic benefit cost $ (108) $ (125) $ (217) $ (250)