Earnings per Common Share Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net Income (Loss) Available to Common Stockholders, Basic [Abstract]
Net income, basic and diluted	$ 160.2	$ 106.5	$ 95.7
Weighted Average Number of Shares Outstanding, Basic and Diluted [Abstract]
Weighted average common shares outstanding for basic earnings per share computation	57,778,857	51,429,366	44,511,774
Dilutive effect of stock-based compensation (in shares)	438,266	473,843	398,622
Weighted average common shares outstanding for diluted earnings per common share computation	58,217,123	51,903,209	44,910,396
Earnings Per Share, Basic and Diluted [Abstract]
Basic earnings per common share (in dollars per share)	$ 2.77	$ 2.07	$ 2.15
Diluted earnings per common share (in dollars per share)	$ 2.75	$ 2.05	$ 2.13
Restricted Stock
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	448	113,874	155,637
Performance Shares
Earnings Per Share, Basic and Diluted [Abstract]
Antidilutive securities excluded from computation of earnings per share (in shares)	83,475	83,635	7,215