Stock-Based Compensation Stock Based Compensation (Details) - USD ($) $ in Millions |
12 Months Ended | ||
|---|---|---|---|
Dec. 31, 2017 |
Dec. 31, 2016 |
Dec. 31, 2015 |
|
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |||
| Pretax compensation expense | $ 48.6 | $ 45.3 | $ 35.2 |
| Income tax benefit | (16.0) | (15.1) | (11.8) |
| Total stock-based compensation expense | 32.6 | 30.2 | 23.4 |
| Stock Awards | |||
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |||
| Pretax compensation expense | 29.4 | 28.1 | 22.5 |
| Income tax benefit | (9.6) | (9.3) | (7.5) |
| Total stock-based compensation expense | 19.8 | 18.8 | 15.0 |
| Stock Options | |||
| Share-based Compensation Arrangement by Share-based Payment Award [Line Items] | |||
| Pretax compensation expense | 19.2 | 17.2 | 12.7 |
| Income tax benefit | (6.4) | (5.8) | (4.3) |
| Total stock-based compensation expense | $ 12.8 | $ 11.4 | $ 8.4 |