Earnings per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Earnings Per Share, Basic and Diluted [Line Items]
Net income	$ 47.1	$ 48.3	$ 141.0	$ 114.3
Weighted average common shares outstanding for basic earnings per share computation (in shares)	61,673,656	63,764,474	61,641,342	64,184,832
Dilutive effects of stock-based compensation (in shares)	74,316	96,983	91,480	110,693
Weighted average common shares outstanding for diluted earnings per common share computation (in shares)	61,747,972	63,861,457	61,732,822	64,295,525
Basic earnings per common share (in dollars per share)	$ 0.76	$ 0.76	$ 2.29	$ 1.78
Diluted earnings per common share (in dollars per share)	$ 0.76	$ 0.76	$ 2.28	$ 1.78
Restricted Stock
Earnings Per Share, Basic and Diluted [Line Items]
Anti-dilutive unvested time restricted stock (in shares)	89,142	68,397	88,677	73,633
Unvested restricted stock
Earnings Per Share, Basic and Diluted [Line Items]
Anti-dilutive unvested time restricted stock (in shares)			355,981	293,236