Retirement benefits - Retirement Cost (Details) - USD ($)
$ in Millions |
3 Months Ended |
Mar. 31, 2023 |
Mar. 31, 2022 |
| Pension plans |
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
| Service cost |
$ 1.8
|
$ 2.1
|
| Interest cost on projected benefit obligation |
12.5
|
9.0
|
| Return on assets – expected |
(14.6)
|
(15.0)
|
| Amortization of losses |
0.9
|
6.3
|
| Settlement loss |
0.1
|
0.4
|
| Net periodic pension cost |
0.7
|
2.8
|
| Retirement benefits other than pensions |
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
| Service cost |
0.1
|
0.0
|
| Interest cost on projected benefit obligation |
4.3
|
3.6
|
| Return on assets – expected |
(2.6)
|
(3.3)
|
| Amortization of losses |
2.8
|
5.0
|
| Amortization of prior service credit |
(2.7)
|
(1.2)
|
| Net periodic pension cost |
1.9
|
4.1
|
| UMWA Plans | Retirement benefits other than pensions |
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
| Service cost |
0.0
|
0.0
|
| Interest cost on projected benefit obligation |
3.0
|
2.7
|
| Return on assets – expected |
(2.6)
|
(3.3)
|
| Amortization of losses |
1.7
|
3.1
|
| Amortization of prior service credit |
(2.7)
|
(1.2)
|
| Net periodic pension cost |
(0.6)
|
1.3
|
| Black Lung and Other Plans | Retirement benefits other than pensions |
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
| Service cost |
0.1
|
0.0
|
| Interest cost on projected benefit obligation |
1.3
|
0.9
|
| Return on assets – expected |
0.0
|
0.0
|
| Amortization of losses |
1.1
|
1.9
|
| Amortization of prior service credit |
0.0
|
0.0
|
| Net periodic pension cost |
2.5
|
2.8
|
| U.S. Plans | Pension plans |
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
| Service cost |
0.0
|
0.0
|
| Interest cost on projected benefit obligation |
8.1
|
5.7
|
| Return on assets – expected |
(11.8)
|
(12.1)
|
| Amortization of losses |
0.5
|
5.8
|
| Settlement loss |
0.0
|
0.0
|
| Net periodic pension cost |
(3.2)
|
(0.6)
|
| Non-U.S. Plans | Pension plans |
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
| Service cost |
1.8
|
2.1
|
| Interest cost on projected benefit obligation |
4.4
|
3.3
|
| Return on assets – expected |
(2.8)
|
(2.9)
|
| Amortization of losses |
0.4
|
0.5
|
| Settlement loss |
0.1
|
0.4
|
| Net periodic pension cost |
$ 3.9
|
$ 3.4
|