Stock-Based Compensation - Unrecognized Compensation Expense (Details) $ in Millions	6 Months Ended
Jun. 30, 2018 USD ($)
Stock options
Stock-Based Compensation
Expected pre-tax stock-based compensation expense	$ 5.2
Weighted average remaining service period	2 years
Restricted stock awards
Stock-Based Compensation
Expected pre-tax stock-based compensation expense	$ 0.5
Weighted average remaining service period	1 year
Restricted stock units
Stock-Based Compensation
Expected pre-tax stock-based compensation expense	$ 11.3
Weighted average remaining service period	2 years 9 months 18 days

Unrecognized cost of unvested share-based compensation awards.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=96867065&loc=d3e5070-113901

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- Definition

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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- Definition

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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