Share-based compensation plans - Compensation Expense (Details) - USD ($)
$ in Millions |
3 Months Ended |
9 Months Ended |
Sep. 30, 2018 |
Sep. 30, 2017 |
Sep. 30, 2018 |
Sep. 30, 2017 |
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
| Share-based payment expense |
$ 6.3
|
$ 4.0
|
$ 18.8
|
$ 12.5
|
| Income tax benefit |
(1.5)
|
(1.5)
|
(4.4)
|
(4.6)
|
| Share-based payment expense, net of tax |
4.8
|
2.5
|
14.4
|
7.9
|
| Performance Shares Units PSU |
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
| Share-based payment expense |
3.1
|
1.9
|
9.7
|
6.4
|
| Market Share Units MSU |
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
| Share-based payment expense |
0.0
|
0.1
|
0.1
|
0.2
|
| Restricted Stock Units |
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
| Share-based payment expense |
1.6
|
1.1
|
4.9
|
3.5
|
| Deferred Stock Units |
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
| Share-based payment expense |
0.4
|
0.3
|
0.9
|
0.8
|
| Stock Options |
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
| Share-based payment expense |
$ 1.2
|
$ 0.6
|
$ 3.2
|
$ 1.6
|