Stock Compensation Expense (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Recorded share-based compensation expense	$ 4.6	$ 9.6	$ 9.1	$ 16.2
Total unrecognized compensation costs related to non-vested share-based compensation	63.9		$ 63.9
Period for recognition			2 years 16 days
Unrecognized compensation expense	$ 31.9		$ 31.9
Common stock issued during period for exercise of stock options, restricted stock grants, and purchases under ESPP (shares)			602,820	1,573,967
Cash received from exercise of stock options and purchases through the ESPP			$ 11.9	$ 35.9
Employee Stock Option
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options granted (in shares)	3,417,600

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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Unrecognized cost of unvested options awarded to employees as compensation.

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Amount of cash inflow from issuance of shares under share-based compensation arrangement. Issuance includes, but is not limited to, exercise of stock options and similar instruments.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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Gross number of share options (or share units) granted during the period.

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Gross number of shares (or other type of equity) issued during the period as a result of any equity-based compensation plan other than an employee stock ownership plan (ESOP). Shares issued could result from the issuance of restricted stock, the exercise of stock options, stock issued under employee stock purchase plans, and/or other employee benefit plans.

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