| Year Ended December 31, | ||||||||||||||||||||
| 2021 | 2020 | 2019 | ||||||||||||||||||
| Basic: | ||||||||||||||||||||
| Net income | $ | 142.8 | $ | 133.3 | $ | 184.4 | ||||||||||||||
| Weighted average number of common shares during the period | 42.0 | 41.9 | 41.7 | |||||||||||||||||
| Net income per share - basic | $ | 3.40 | $ | 3.18 | $ | 4.42 | ||||||||||||||
| Diluted: | ||||||||||||||||||||
| Net income | $ | 142.8 | $ | 133.3 | $ | 184.4 | ||||||||||||||
| Weighted average number of common shares during the period | 42.0 | 41.9 | 41.7 | |||||||||||||||||
Weighted average number of common equivalent shares to reflect the dilutive effect of common stock equivalent securities: | ||||||||||||||||||||
| Stock options | — | — | 0.1 | |||||||||||||||||
| Common stock units related to deferred compensation for employees | — | — | — | |||||||||||||||||
| Restricted common stock units related to incentive compensation | 0.2 | 0.1 | 0.1 | |||||||||||||||||
Total common and common equivalent shares adjusted to calculate diluted earnings per share | 42.2 | 42.0 | 41.9 | |||||||||||||||||
| Net income per share – diluted | $ | 3.39 | $ | 3.17 | $ | 4.40 | ||||||||||||||
| Percentage of dilution compared to basic net income per share | 0.3 | % | 0.3 | % | 0.5 | % | ||||||||||||||