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Fair Value Measurements - Reconciliation Of Level 3 Assets (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Beginning of measurement period $ 714,017 $ 649,688
Actual return on plan assets:    
End of measurement period 741,191 714,017
U.S.    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Beginning of measurement period 403,144  
Actual return on plan assets:    
End of measurement period 404,275 403,144
U.K Plan    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Beginning of measurement period 296,943  
Actual return on plan assets:    
End of measurement period 303,954 296,943
Significant Unobservable Inputs (Level 3) | U.S.    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Beginning of measurement period 14,766 16,488
Actual return on plan assets:    
Related to assets still held at the reporting date 1,172 (1,722)
End of measurement period 15,938 14,766
Significant Unobservable Inputs (Level 3) | U.K Plan    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Beginning of measurement period 9,735 9,945
Actual return on plan assets:    
Related to assets still held at the reporting date (163) (210)
End of measurement period $ 9,572 $ 9,735