Earnings per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Earnings Per Share, Basic and Diluted [Line Items]
Net income	$ 37.9	$ 41.7	$ 79.5	$ 78.4
Weighted average common shares outstanding for basic earnings per share computation (in shares)	64,504,546	56,335,186	62,419,442	56,288,453
Dilutive effects of stock-based compensation (in shares)	202,913	363,835	248,949	381,947
Weighted average common shares outstanding for diluted earnings per common share computation (in shares)	64,707,459	56,699,021	62,668,391	56,670,400
Basic earnings per common share (in dollars per share)	$ 0.59	$ 0.74	$ 1.27	$ 1.39
Diluted earnings per common share (in dollars per share)	$ 0.59	$ 0.74	$ 1.27	$ 1.38
Restricted Stock
Earnings Per Share, Basic and Diluted [Line Items]
Anti-dilutive unvested time restricted stock (in shares)	761	85	108,046	128,461
Unvested restricted stock
Earnings Per Share, Basic and Diluted [Line Items]
Anti-dilutive unvested time restricted stock (in shares)			147,415	159,812