Stock Based Compensation - Stock Options and Restricted Stock Units Narrative (Details) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2018 USD ($) $ / shares
Options to purchase common stock
Stock-based Compensation
Weighted-average grant-date fair value (in dollars per share) \| $ / shares	$ 12.86	$ 19.07
Total unrecognized compensation expense	$ 49.3	$ 49.3
Weighted average period over which unrecognized compensation is expected to be recognized		2 years 234 days
RSUs
Stock-based Compensation
Total unrecognized compensation expense	$ 7.3	$ 7.3
Weighted average period over which unrecognized compensation is expected to be recognized		2 years 307 days

Unrecognized cost of unvested share-based compensation awards.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=109197908&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.

+ References

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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.

+ References

No definition available.

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (d)(1)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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