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, 2018	Sep. 30, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income, basic and diluted	$ 160.2		$ 181.0	$ 160.2	$ 106.5
Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract]
Weighted average common shares outstanding for basic earnings per share computation			63,645,029	57,778,857	51,429,366
Dilutive effect of stock-based compensation (in shares)			239,839	438,266	473,843
Weighted average common shares outstanding for diluted earnings per common share computation			63,884,868	58,217,123	51,903,209
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per common share (in dollars per share)			$ 2.84	$ 2.77	$ 2.07
Diluted earnings per common share (in dollars per share)			$ 2.83	$ 2.75	$ 2.05
Restricted Stock [Member]
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)		448	150		113,874
Performance Shares
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)			138,298	83,475	83,635