Earnings per Common Share - Computation of Basic and Diluted Earnings per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Earnings Per Share, Basic and Diluted [Line Items]
Net income	$ 41.6	$ 36.7
Weighted average common shares outstanding for basic earnings per share computation	60,311,171	56,241,201
Dilutive effects of stock-based compensation	286,899	410,977
Weighted average common shares outstanding for diluted earnings per common share computation	60,598,070	56,652,178
Basic earnings per common share (in dollars per share)	$ 0.69	$ 0.65
Diluted earnings per common share (in dollars per share)	$ 0.69	$ 0.65
Restricted Stock [Member]
Earnings Per Share, Basic and Diluted [Line Items]
Anti-dilutive unvested time restricted stock	121,452	139,943
Restricted stock
Earnings Per Share, Basic and Diluted [Line Items]
Anti-dilutive unvested time restricted stock	154,387	164,019
Class A Common Stock
Earnings Per Share, Basic and Diluted [Line Items]
Common Stock, Shares, Outstanding	38,461,948		38,169,575
Class B Common Stock
Earnings Per Share, Basic and Diluted [Line Items]
Common Stock, Shares, Outstanding	22,374,171		22,453,672

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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Number of shares of common stock outstanding. Common stock represent the ownership interest in a corporation.

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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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