Share-based compensation plans - Compensation Expense (Details) - USD ($)
$ in Millions |
3 Months Ended |
9 Months Ended |
Sep. 30, 2016 |
Sep. 30, 2015 |
Sep. 30, 2016 |
Sep. 30, 2015 |
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
| Share-based payment expense |
$ 1.8
|
$ 2.5
|
$ 6.7
|
$ 11.0
|
| Income tax benefit |
(0.6)
|
(0.8)
|
(2.3)
|
(3.7)
|
| Share-based payment expense, net of tax |
1.2
|
1.7
|
4.4
|
7.3
|
| Performance Shares |
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
| Share-based payment expense |
0.6
|
1.6
|
2.9
|
6.1
|
| Market Share Units MSU |
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
| Share-based payment expense |
(0.2)
|
0.2
|
0.0
|
2.1
|
| Restricted Stock |
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
| Share-based payment expense |
0.9
|
0.6
|
2.9
|
2.5
|
| Deferred Stock Units |
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
| Share-based payment expense |
0.3
|
0.1
|
0.6
|
0.3
|
| Stock Options |
|
|
|
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] |
|
|
|
|
| Share-based payment expense |
$ 0.2
|
$ 0.0
|
$ 0.3
|
$ 0.0
|