Share-Based Compensation - Compensation Expense for Restricted Stock and Performance Units (Details) - USD ($) $ in Millions |
3 Months Ended | 6 Months Ended | ||
|---|---|---|---|---|
Jun. 30, 2017 |
Jun. 30, 2016 |
Jun. 30, 2017 |
Jun. 30, 2016 |
|
| Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] | ||||
| Total share-based compensation expense | $ 5.6 | $ 4.7 | $ 10.2 | $ 10.3 |
| Income tax benefit | (2.1) | (1.8) | (3.9) | (4.0) |
| Share-based compensation expense, net of tax benefit | 3.5 | 2.9 | 6.3 | 6.3 |
| Restricted Stock | ||||
| Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] | ||||
| Total share-based compensation expense | 4.1 | 3.9 | 7.5 | 8.8 |
| Performance Units | ||||
| Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Line Items] | ||||
| Total share-based compensation expense | $ 1.5 | $ 0.8 | $ 2.7 | $ 1.5 |