Stock Plans and Stock-Based Compensation - Stock Options - Additional Information (Details) - USD ($) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 7,785	$ 6,694
Intrinsic value of options exercised	22,200
Intrinsic value of all outstanding options	$ 62,200
Stock option granted during period	0		557,896
Total unrecognized compensation cost	$ 11,100
Weighted-average amortization period	2 years 2 months 4 days
Stock Options
Share Based Compensation Arrangement By Share Based Payment Award [Line Items]
Stock-based compensation expense	$ 1,900	$ 1,900
Weighted-average amortization period	2 years 5 months 12 days

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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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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Unrecognized cost of unvested options awarded to employees as compensation.

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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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Amount of accumulated difference between fair value of underlying shares on dates of exercise and exercise price on options exercised (or share units converted) into shares.

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- Definition

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

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Amount by which the current fair value of the underlying stock exceeds the exercise price of options outstanding.

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