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Fair Value Measurements - Reconciliation of Level 3 Assets (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2023
Dec. 31, 2022
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Beginning of measurement period $ 477,728 $ 739,789
Actual return on plan assets:    
End of measurement period 443,157 477,728
U.K Plan    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Beginning of measurement period 174,873  
Actual return on plan assets:    
End of measurement period 156,142 174,873
U.S.    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Beginning of measurement period 281,159  
Actual return on plan assets:    
End of measurement period 271,412 281,159
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 9,475 11,255
Actual return on plan assets:    
Related to assets still held at the reporting date (4,548) (1,780)
Purchases, sales and settlements, net 0 0
End of measurement period 4,927 9,475
Significant Unobservable Inputs (Level 3) | U.S.    
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]    
Beginning of measurement period 18,194 19,268
Actual return on plan assets:    
Related to assets still held at the reporting date (6,929) (1,074)
Purchases, sales and settlements, net 34 0
End of measurement period $ 11,299 $ 18,194