Earnings per Common Share - Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Earnings Per Share, Basic and Diluted [Line Items]
Net income	$ 41.7	$ 21.8	$ 78.4	$ 45.0
Weighted average common shares outstanding for basic earnings per share computation	56,335,186	47,612,971	56,288,453	46,170,500
Dilutive effects of stock-based compensation	363,835	461,480	381,947	510,875
Weighted average common shares outstanding for diluted earnings per common share computation	56,699,021	48,074,451	56,670,400	46,681,375
Basic earnings per common share (in dollars per share)	$ 0.74	$ 0.46	$ 1.39	$ 0.97
Diluted earnings per common share (in dollars per share)	$ 0.74	$ 0.45	$ 1.38	$ 0.96
Unvested Restricted Stock
Earnings Per Share, Basic and Diluted [Line Items]
Anti-dilutive unvested time restricted stock	85	96,262	128,461	92,626
Restricted stock
Earnings Per Share, Basic and Diluted [Line Items]
Anti-dilutive unvested time restricted stock			159,812	182,835

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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Additional shares included in the calculation of diluted EPS as a result of the potentially dilutive effect of share based payment arrangements using the treasury stock method.

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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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Number of [basic] shares or units, after adjustment for contingently issuable shares or units and other shares or units not deemed outstanding, determined by relating the portion of time within a reporting period that common shares or units have been outstanding to the total time in that period.

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