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Benefit Plans - Reconciliation of Funded Status (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Change in plan assets      
Fair value of assets at beginning of year $ 260,747    
Fair value of plan assets at end of year 297,603 $ 260,747  
Pension Plan      
Change in benefit obligation      
Benefit obligation at beginning of year 338,240 187,877  
Service cost 8,659 5,947 $ 5,790
Interest cost 10,484 8,506 6,879
Business combination 0 108,162  
Actuarial (gain)/loss 42,164 35,502  
Implicit rate subsidy 0 0  
Plan participants contributions 0 0  
Administrative expenses paid (163) (10)  
Benefits paid (13,293) (7,744)  
Benefit obligation at end of year 386,091 338,240 187,877
Change in plan assets      
Fair value of assets at beginning of year 243,521 127,610  
Actual return on plan assets 32,869 34,807  
Business combination 0 79,382  
Employer contributions 16,609 9,476  
Plan participants contributions 0 0  
Administrative expenses paid (163) (10)  
Benefits paid (14,712) (7,744)  
Fair value of plan assets at end of year 278,124 243,521 127,610
Funded status at end of year (107,967) (94,719)  
Other Postretirement Benefits Plan      
Change in benefit obligation      
Benefit obligation at beginning of year 32,546 17,489  
Service cost 900 581 616
Interest cost 956 775 627
Business combination 0 12,537  
Actuarial (gain)/loss 2,421 2,027  
Implicit rate subsidy (211) (217)  
Plan participants contributions 254 85  
Administrative expenses paid 0 0  
Benefits paid (1,072) (731)  
Benefit obligation at end of year 35,794 32,546 17,489
Change in plan assets      
Fair value of assets at beginning of year 17,226 5,849  
Actual return on plan assets 2,383 1,972  
Business combination 0 9,314  
Employer contributions 675 738  
Plan participants contributions 254 85  
Administrative expenses paid (54) (55)  
Benefits paid (1,005) (677)  
Fair value of plan assets at end of year 19,479 17,226 $ 5,849
Funded status at end of year $ (16,315) $ (15,320)