Earnings per Common Share Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Mar. 31, 2021	Sep. 30, 2020	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income, basic and diluted			$ 192.1	$ 161.2	$ 181.0
Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract]
Weighted average common shares outstanding for basic earnings per share computation			61,650,312	63,611,891	63,645,029
Dilutive effect of stock-based compensation (in shares)			91,516	117,579	239,839
Weighted average common shares outstanding for diluted earnings per common share computation			61,741,828	63,729,470	63,884,868
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per common share (in dollars per share)			$ 3.12	$ 2.53	$ 2.84
Diluted earnings per common share (in dollars per share)			$ 3.11	$ 2.53	$ 2.83
Restricted Stock [Member]
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)		3,094	83,952		138,298
Performance Shares
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	291,540		333,767		150