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Benefit Plans - Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Pension Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service cost $ 8,659 $ 5,947 $ 5,790
Interest cost 10,484 8,506 6,879
Expected return on assets (15,715) (10,118) (9,255)
Amortization of prior service cost 41 48 51
Recognized actuarial loss 4,971 3,984 3,986
Net periodic benefit cost 8,440 8,367 7,451
Other Postretirement Benefits Plan      
Defined Benefit Plan Disclosure [Line Items]      
Service cost 900 581 616
Interest cost 956 775 627
Expected return on assets (857) (475) (450)
Amortization of prior service cost 97 197 197
Recognized actuarial loss 229 240 321
Net periodic benefit cost $ 1,325 $ 1,318 $ 1,311