v3.26.1
Retirement benefits - Retirement Cost (Details) - USD ($)
$ in Millions
 3 Months Ended
Mar. 31, 2026
Mar. 31, 2025
Pension plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 2.4 $ 2.0
Interest cost on projected benefit obligation 12.4 12.1
Return on assets – expected (12.4) (13.8)
Amortization of losses 3.4 1.9
Amortization of prior service cost (credit) 0.0 0.1
Settlement loss 0.0 0.3
Net periodic pension cost (credit) 5.8 2.6
Retirement benefits other than pensions    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 0.1 0.0
Interest cost on projected benefit obligation 3.4 3.4
Return on assets – expected (2.4) (2.4)
Amortization of losses 1.2 1.2
Amortization of prior service cost (credit) (1.4) (2.6)
Net periodic pension cost (credit) 0.9 (0.4)
Retirement benefits other than pensions | UMWA Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 0.0 0.0
Interest cost on projected benefit obligation 2.4 2.3
Return on assets – expected (2.4) (2.4)
Amortization of losses 0.5 0.3
Amortization of prior service cost (credit) (1.4) (2.6)
Net periodic pension cost (credit) (0.9) (2.4)
Retirement benefits other than pensions | Black Lung and Other Plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 0.1 0.0
Interest cost on projected benefit obligation 1.0 1.1
Return on assets – expected 0.0 0.0
Amortization of losses 0.7 0.9
Amortization of prior service cost (credit) 0.0 0.0
Net periodic pension cost (credit) 1.8 2.0
U.S. Plans | Pension plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 0.0 0.0
Interest cost on projected benefit obligation 7.4 7.9
Return on assets – expected (9.4) (11.1)
Amortization of losses 2.4 1.3
Amortization of prior service cost (credit) 0.0 0.0
Settlement loss 0.0 0.0
Net periodic pension cost (credit) 0.4 (1.9)
Non-U.S. Plans | Pension plans    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 2.4 2.0
Interest cost on projected benefit obligation 5.0 4.2
Return on assets – expected (3.0) (2.7)
Amortization of losses 1.0 0.6
Amortization of prior service cost (credit) 0.0 0.1
Settlement loss 0.0 0.3
Net periodic pension cost (credit) $ 5.4 $ 4.5