Share-based Compensation - Additional Information (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
Disclosure Of Compensation Related Costs Sharebased Payments [Abstract]
Share-based compensation expense recognized	$ 5.2	$ 11.7	$ 11.1	$ 19.6
Weighted average fair value of options granted (in dollars per share)	$ 14.32	$ 7.24	$ 12.84	$ 10.75
Share based compensation, other disclosures
Total unrecognized compensation costs related to unvested stock options and restricted stock awards that the company expects to recognize	$ 30.3		$ 30.3
Weighted average period			1 year 9 months 18 days
Repurchases of common stock, shares	16,339		63,124
Average price of common stock per share	$ 23.58		$ 25.15
Repurchases of common stock	$ 0.4		$ 1.6

Represents the expense recognized during the period arising from equity-based compensation arrangements (for example, shares of stock, unit, stock options or other equity instruments) with employees, directors and certain consultants qualifying for treatment as employees.

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Unrecognized cost of unvested share-based compensation awards.

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Weighted average period over which unrecognized compensation is expected to be recognized for equity-based compensation plans, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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The weighted average grant-date fair value of options granted during the reporting period as calculated by applying the disclosed option pricing methodology.

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Total cost of shares repurchased divided by the total number of shares repurchased.

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Number of shares that have been repurchased during the period and are being held in treasury.

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Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method.

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