Stock-Based Compensation - Employee Stock Purchase Plan (Details) - Employee Stock Purchase Plan - shares	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-Based Compensation
Common stock reserved for issuance (in shares)	26,667
Purchase price of fair value of common stock (in percent)	85.00%
Issued (in shares)	9,524	5,359	3,956
Available for issuance (in shares)	3,666

Aggregate number of common shares reserved for future issuance.

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-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
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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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The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

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-Publisher FASB
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Purchase price of common stock expressed as a percentage of its fair value.

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

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