Shares used to calculate earnings per share (Details) - shares shares in Millions |
3 Months Ended | |
|---|---|---|
Mar. 31, 2018 |
Mar. 31, 2017 |
|
| Earnings Per Share [Abstract] | ||
| Basic (shares) | 50.9 | 50.5 |
| Effect of dilutive stock options and awards (shares) | 1.2 | 1.0 |
| Diluted (shares) | 52.1 | 51.5 |
| Antidilutive stock options and awards excluded from denominator (shares) | 0.0 | 0.0 |
| Share-based goods and nonemployee services transaction, quantity of securities issued (shares) | 0.3 | 0.4 |