Retirement benefits - Retirement Cost (Details) - USD ($)
$ in Millions |
3 Months Ended |
9 Months Ended |
Sep. 30, 2018 |
Sep. 30, 2017 |
Sep. 30, 2018 |
Sep. 30, 2017 |
| Pension Plan |
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
| Service cost |
$ 2.6
|
$ 2.8
|
$ 8.2
|
$ 8.5
|
| Interest cost on projected benefit obligation |
10.5
|
12.0
|
33.8
|
38.7
|
| Return on assets – expected |
(16.1)
|
(15.9)
|
(48.6)
|
(47.3)
|
| Amortization of losses |
7.8
|
7.7
|
24.1
|
22.7
|
| Amortization of prior service cost |
0.0
|
0.4
|
0.2
|
0.8
|
| Settlement loss |
0.4
|
0.6
|
1.4
|
1.4
|
| Net periodic pension cost |
5.2
|
7.6
|
19.1
|
24.8
|
| Other Postretirement Benefits Plan |
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
| Service cost |
|
|
0.1
|
0.1
|
| Interest cost on projected benefit obligation |
4.9
|
5.5
|
15.2
|
16.1
|
| Return on assets – expected |
(4.1)
|
(4.1)
|
(12.5)
|
(12.4)
|
| Amortization of losses |
6.5
|
6.2
|
19.7
|
17.6
|
| Amortization of prior service cost |
(1.0)
|
(0.7)
|
(2.7)
|
(2.2)
|
| Net periodic pension cost |
6.3
|
6.9
|
19.8
|
19.2
|
| U.S. Plans | Pension Plan |
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
| Service cost |
0.0
|
0.0
|
0.0
|
0.0
|
| Interest cost on projected benefit obligation |
8.0
|
8.8
|
24.0
|
26.4
|
| Return on assets – expected |
(13.4)
|
(13.3)
|
(40.2)
|
(39.9)
|
| Amortization of losses |
6.8
|
6.3
|
20.8
|
18.7
|
| Amortization of prior service cost |
0.0
|
0.0
|
0.0
|
0.0
|
| Settlement loss |
0.0
|
0.0
|
0.0
|
0.0
|
| Net periodic pension cost |
1.4
|
1.8
|
4.6
|
5.2
|
| Non-U.S. Plans | Pension Plan |
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
| Service cost |
2.6
|
2.8
|
8.2
|
8.5
|
| Interest cost on projected benefit obligation |
2.5
|
3.2
|
9.8
|
12.3
|
| Return on assets – expected |
(2.7)
|
(2.6)
|
(8.4)
|
(7.4)
|
| Amortization of losses |
1.0
|
1.4
|
3.3
|
4.0
|
| Amortization of prior service cost |
0.0
|
0.4
|
0.2
|
0.8
|
| Settlement loss |
0.4
|
0.6
|
1.4
|
1.4
|
| Net periodic pension cost |
3.8
|
5.8
|
14.5
|
19.6
|
| UMWA Plans | Other Postretirement Benefits Plan |
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
| Service cost |
|
|
0.0
|
0.0
|
| Interest cost on projected benefit obligation |
4.2
|
4.6
|
12.9
|
13.7
|
| Return on assets – expected |
(4.1)
|
(4.1)
|
(12.5)
|
(12.4)
|
| Amortization of losses |
4.9
|
5.2
|
15.4
|
14.6
|
| Amortization of prior service cost |
(1.2)
|
(1.2)
|
(3.5)
|
(3.5)
|
| Net periodic pension cost |
3.8
|
4.5
|
12.3
|
12.4
|
| Black Lung and Other Plans | Other Postretirement Benefits Plan |
|
|
|
|
| Defined Benefit Plan Disclosure [Line Items] |
|
|
|
|
| Service cost |
|
|
0.1
|
0.1
|
| Interest cost on projected benefit obligation |
0.7
|
0.9
|
2.3
|
2.4
|
| Return on assets – expected |
0.0
|
0.0
|
0.0
|
0.0
|
| Amortization of losses |
1.6
|
1.0
|
4.3
|
3.0
|
| Amortization of prior service cost |
0.2
|
0.5
|
0.8
|
1.3
|
| Net periodic pension cost |
$ 2.5
|
$ 2.4
|
$ 7.5
|
$ 6.8
|