Stockholders' Equity - Summary of activity under Employee Stock Purchase Plan (Details) - Employee Stock Purchase Plan - shares	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Summary of the Employee Stock Purchase Plan activity
Outstanding and exercisable, beginning of period (shares)	328,533	318,100
Granted (shares)	235,572	203,836
Exercised (shares)	(14,630)	(8,835)
Forfeited (shares)	(16,625)	(10,580)
Expired (shares)	(142,074)	(135,790)
Outstanding and exercisable, end of period (shares)	390,776	366,731

Number of options or other stock instruments for which the right to exercise has lapsed under the terms of the plan agreements.

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- Definition

The number of shares under options that were cancelled during the reporting period as a result of occurrence of a terminating event specified in contractual agreements pertaining to the stock option plan.

+ References

+ Details

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- Definition

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

+ References

No definition available.

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- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

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- Definition

Number of share options (or share units) exercised during the current period.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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