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Retirement Benefits - Narrative (Details)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
USD ($)
employee
Dec. 31, 2019
USD ($)
Dec. 31, 2018
USD ($)
Defined Contribution Plan Disclosure [Line Items]      
Employer matching contributions | $ $ 3,250 $ 2,842 $ 2,700
Number of employees 2,600    
Entity number of employees, field personnel 2,200    
Union Members      
Defined Contribution Plan Disclosure [Line Items]      
Number of employees 300    
Number of employees, percent 13.00%