Basis of Presentation and Significant Accounting Policies - Calculation of Earnings Per Share, Basic and Diluted (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Basic:
Net income for the period	$ 142.8	$ 133.3	$ 184.4
Weighted average number of common shares during the period (in shares)	42.0	41.9	41.7
Net income per share - basic (in usd per share)	$ 3.40	$ 3.18	$ 4.42
Diluted:
Net income for the period	$ 142.8	$ 133.3	$ 184.4
Weighted average number of common shares during the period (in shares)	42.0	41.9	41.7
Weighted average number of common equivalent shares to reflect the dilutive effect of common stock equivalent securities (in millions):
Stock options (in shares)	0.0	0.0	0.1
CSUs related to deferred compensation for employees (in shares)	0.0	0.0	0.0
RSUs related to incentive compensation (in shares)	0.2	0.1	0.1
Total common and common equivalent shares adjusted to calculate diluted earnings per share (in shares)	42.2	42.0	41.9
Net income per share - diluted (in usd per share)	$ 3.39	$ 3.17	$ 4.40