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, 2020	Sep. 30, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income, basic and diluted			$ 161.2	$ 181.0	$ 160.2
Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract]
Weighted average common shares outstanding for basic earnings per share computation			63,611,891	63,645,029	57,778,857
Dilutive effect of stock-based compensation (in shares)			117,579	239,839	438,266
Weighted average common shares outstanding for diluted earnings per common share computation			63,729,470	63,884,868	58,217,123
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per common share (in dollars per share)			$ 2.53	$ 2.84	$ 2.77
Diluted earnings per common share (in dollars per share)			$ 2.53	$ 2.83	$ 2.75
Restricted Stock [Member]
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)		150	3,094		83,475
Performance Shares
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	138,298		291,540		448