Earnings per Common Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Earnings Per Share, Basic and Diluted [Line Items]
Net income	$ 49.1	$ 41.4	$ 128.6	$ 119.8
Weighted average common shares outstanding for basic earnings per share computation (in shares)	64,832,324	58,254,575	63,232,575	56,951,029
Dilutive effects of stock-based compensation (in shares)	211,162	385,900	238,708	378,998
Weighted average common shares outstanding for diluted earnings per common share computation (in shares)	65,043,486	58,640,475	63,471,283	57,330,027
Basic earnings per common share (in dollars per share)	$ 0.76	$ 0.71	$ 2.03	$ 2.10
Diluted earnings per common share (in dollars per share)	$ 0.76	$ 0.71	$ 2.03	$ 2.09
Restricted Stock
Earnings Per Share, Basic and Diluted [Line Items]
Anti-dilutive unvested time restricted stock (in shares)	4,808	4,376	5,569	120,553
Unvested restricted stock
Earnings Per Share, Basic and Diluted [Line Items]
Anti-dilutive unvested time restricted stock (in shares)			143,415	154,176

Earnings Per Share, Basic and Diluted [Line Items]

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Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

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The amount of net income (loss) for the period per each share of common stock or unit outstanding during the reporting period.

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The amount of net income (loss) for the period available to each share of common stock or common unit outstanding during the reporting period and to each share or unit that would have been outstanding assuming the issuance of common shares or units for all dilutive potential common shares or units outstanding during the reporting period.

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The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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