| Three Months Ended September 30, | Nine Months Ended September 30, | |||||||||||||||||||||||||
| 2021 | 2020 | 2021 | 2020 | |||||||||||||||||||||||
| Basic: | ||||||||||||||||||||||||||
| Net income | $ | 16.3 | $ | 36.5 | $ | 102.3 | $ | 85.5 | ||||||||||||||||||
| Weighted average number of common shares during the period | 42.0 | 41.9 | 42.0 | 41.9 | ||||||||||||||||||||||
| Net income per share – basic | $ | 0.39 | $ | 0.87 | $ | 2.44 | $ | 2.04 | ||||||||||||||||||
| Diluted: | ||||||||||||||||||||||||||
| Net income | $ | 16.3 | $ | 36.5 | $ | 102.3 | $ | 85.5 | ||||||||||||||||||
| Weighted average number of common shares during the period | 42.0 | 41.9 | 42.0 | 41.9 | ||||||||||||||||||||||
| Weighted average number of common equivalent shares to reflect the dilutive effect of common stock equivalent securities: | ||||||||||||||||||||||||||
| Stock options | 0.1 | 0.1 | 0.1 | — | ||||||||||||||||||||||
| Common stock units related to deferred compensation for employees | — | — | — | — | ||||||||||||||||||||||
| Restricted common stock units related to incentive compensation | 0.1 | 0.1 | 0.1 | 0.1 | ||||||||||||||||||||||
Total common and common equivalent shares adjusted to calculate diluted earnings per share | 42.2 | 42.1 | 42.2 | 42.0 | ||||||||||||||||||||||
| Net income per share – diluted | $ | 0.39 | $ | 0.87 | $ | 2.43 | $ | 2.03 | ||||||||||||||||||