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Reclassifications out of Accumulated Other Comprehensive Loss - Summary of Reclassifications out of Accumulated Other Comprehensive Loss (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2025
Dec. 31, 2024
Dec. 31, 2023
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]      
Amortization of prior service cost $ 0.0 $ 8.4 $ 0.5
Amortization of actuarial net losses/(gains) 0.0 (0.2) $ (2.1)
Total reclassifications (0.0) (98.5)  
Defined Benefit Pension Plan Items [Member]      
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]      
Total reclassifications 0.0 (98.5)  
Reclassification Out of Accumulated Other Comprehensive Income/(Loss) [Member]      
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]      
Total before tax 0.0 146.3  
Income tax expense 0.0 (47.8)  
Total reclassifications 0.0 (98.5)  
Reclassification Out of Accumulated Other Comprehensive Income/(Loss) [Member] | Defined Benefit Pension Plan Items [Member]      
Reclassification Adjustment out of Accumulated Other Comprehensive Income [Line Items]      
Amortization of prior service cost 0.0 8.4  
Amortization of actuarial net losses/(gains) $ 0.0 (0.2)  
Pension scheme settlement   $ 138.1