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Employee Benefit Plans - Pension Plans Narrative (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
USD ($)
pensionPlan
Dec. 31, 2019
USD ($)
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets $ 403,500 $ 351,800
Benefit obligation 558,400 509,400
Items not yet recognized as net periodic pension cost (benefit) 165,900 191,600
Loss on plan obligations 27,700  
Net actuarial loss due to assumption changes 37,100  
Net gains due to plan experience $ 9,500  
Number of Defined Benefit Plans | pensionPlan 2  
Pension Plan    
Defined Benefit Plan Disclosure [Line Items]    
Fair value of plan assets $ 378,900 331,400
Benefit obligation 470,100 439,400
Unfunded status of plan $ 91,200 $ 108,000