v3.25.2
Retirement benefits - Retirement Cost (Details) - USD ($)
$ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2025
Jun. 30, 2024
Jun. 30, 2025
Jun. 30, 2024
Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 2.1 $ 2.2 $ 4.1 $ 4.4
Interest cost on projected benefit obligation 12.3 12.1 24.4 24.3
Return on assets – expected (14.1) (14.4) (27.9) (28.9)
Amortization of losses 2.1 1.9 4.0 4.0
Amortization of prior service cost (credit)     0.1 0.0
Settlement loss 0.4 0.0 0.7 0.3
Net periodic pension cost (credit) 2.8 1.8 5.4 4.1
Retirement benefits other than pensions        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 0.1 0.0 0.1 0.1
Interest cost on projected benefit obligation 3.6 3.7 7.0 7.6
Return on assets – expected (2.5) (2.6) (4.9) (5.1)
Amortization of losses 1.4 1.1 2.6 3.5
Amortization of prior service cost (credit) (2.5) (1.5) (5.1) (4.3)
Net periodic pension cost (credit) 0.1 0.7 (0.3) 1.8
Retirement benefits other than pensions | UMWA Plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 0.0 0.0 0.0 0.0
Interest cost on projected benefit obligation 2.5 2.6 4.8 5.2
Return on assets – expected (2.5) (2.6) (4.9) (5.1)
Amortization of losses 0.6 0.1 0.9 1.4
Amortization of prior service cost (credit) (2.5) (1.6) (5.1) (4.4)
Net periodic pension cost (credit) (1.9) (1.5) (4.3) (2.9)
Retirement benefits other than pensions | Black Lung and Other Plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 0.1 0.0 0.1 0.1
Interest cost on projected benefit obligation 1.1 1.1 2.2 2.4
Return on assets – expected 0.0 0.0 0.0 0.0
Amortization of losses 0.8 1.0 1.7 2.1
Amortization of prior service cost (credit) 0.0 0.1 0.0 0.1
Net periodic pension cost (credit) 2.0 2.2 4.0 4.7
U.S. Plans | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 0.0 0.0 0.0 0.0
Interest cost on projected benefit obligation 7.8 7.7 15.7 15.4
Return on assets – expected (11.2) (11.6) (22.3) (23.2)
Amortization of losses 1.4 1.2 2.7 2.7
Amortization of prior service cost (credit)     0.0 0.0
Settlement loss 0.0 0.0 0.0 0.0
Net periodic pension cost (credit) (2.0) (2.7) (3.9) (5.1)
Non-U.S. Plans | Pension plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 2.1 2.2 4.1 4.4
Interest cost on projected benefit obligation 4.5 4.4 8.7 8.9
Return on assets – expected (2.9) (2.8) (5.6) (5.7)
Amortization of losses 0.7 0.7 1.3 1.3
Amortization of prior service cost (credit)     0.1 0.0
Settlement loss 0.4 0.0 0.7 0.3
Net periodic pension cost (credit) $ 4.8 $ 4.5 $ 9.3 $ 9.2