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.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
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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

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
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- Definition

Unrecognized cost of unvested options awarded to employees as compensation.

+ References

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- Definition

Discount rate from fair value on purchase date that participants pay for shares.

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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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No definition available.

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- Definition

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

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No definition available.

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- Definition

Number of options outstanding, including both vested and non-vested options.

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Number of non-vested options outstanding.

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No definition available.

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Number of options vested.

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- Definition

Number of shares issued during the period as a result of an employee stock purchase plan.

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Equity - Narrative (Detail) - USD ($) $ in Millions			3 Months Ended		6 Months Ended
Equity - Narrative (Detail) - USD ($) $ in Millions	May 25, 2017	May 24, 2017	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock option outstanding (in shares)			7,455,054		7,455,054		6,342,526
Stock options, granted (in shares)					2,168,602
Unrecognized stock-based compensation cost			$ 4.5		$ 4.5
Unrecognized share-based compensation expense, weighted-average recognition period (in years)					9 months
Employee Stock Purchase Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Change in discount rate	15.00%	5.00%
Shares purchased by employees under ESPP			29,266	26,569	66,188	50,932
Stock compensation expense (gain)			$ 0.1	$ 0.0	$ 0.1	$ 0.0
Non Employee
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock options, granted (in shares)					840,001
Stock option vested (in shares)					416,663
Stock option unvested (in shares)			423,338		423,338