Stock Option Plan - Activity Under Compensation Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Shares Available for Grant
Beginning balance (in shares)	57,889	328,290	27,605
Authorized (in shares)	0		223,664	1,421,867
Options granted (in shares)	(32,950)		(629,716)	(1,375,329)
Options cancelled (in shares)	(7,387)		(135,651)	(254,147)
Ending balance (in shares)	32,326	57,889	328,290	27,605
Number of Shares
Beginning balance (in shares)	4,364,377	4,308,890	3,478,346
Options granted (in shares)	(32,950)		(629,716)	(1,375,329)
Options exercised (in shares)	(251,305)		(438,578)	(290,638)
Options cancelled (in shares)	(7,387)		(135,651)	(254,147)
Ending balance (in shares)	4,138,635	4,364,377	4,308,890	3,478,346
Weighted Average Exercise Price
Beginning balance (in USD per share)	$ 3.79	$ 3.09	$ 1.46
Options granted (in USD per share)	11.29		6.51	$ 6.54
Options exercised (in USD per share)	1.50		1.50	1.17
Options cancelled (in USD per share)	1.56		1.56	1.55
Ending balance (in USD per share)	$ 4.00	$ 3.79	$ 3.09	$ 1.46
Weighted Average Remaining Contractual Term (in Years), awards outstanding	7 years 3 months 3 days	7 years 4 months 9 days	7 years 9 months 21 days	7 years 8 months 15 days
Aggregate Intrinsic Value, awards outstanding	$ 30,602	$ 33,132	$ 5,073	$ 444
Number of Shares, vested and exercisable (in shares)	2,451,044	2,459,116
Weighted Average Exercise Price, vested and exercisable (in USD per share)	$ 2.70	$ 2.35
Weighted Average Remaining Contractual Term (in Years), vested and exercisable	6 years 4 months 9 days	6 years 3 months 10 days
Aggregate Intrinsic Value, vested and exercisable	$ 21,195	$ 22,109
Number of Shares, vested and expect to vest (in shares)	4,138,635	4,464,377
Weighted Average Exercise Price, vested and expected to vest (in dollars per share)	$ 4.00	$ 3.79
Weighted Average Remaining Contractual Term (in Years), vested and expected to vest	7 years 3 months 3 days	7 years 4 months 9 days
Aggregate Intrinsic Value, vested and expected to vest	$ 30,602	$ 33,132

Share-based Compensation Arrangement by Share-based Payment Award, Awards Available for Grant [Roll Forward]

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- Definition

Number of additional shares authorized for issuance under an established share-based compensation plan.

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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.

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The number of shares into which fully or partially vested stock options outstanding as of the balance sheet date can be currently converted under the option plan.

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The weighted-average price as of the balance sheet date at which grantees can acquire the shares reserved for issuance on vested portions of options outstanding and currently exercisable under the stock option plan.

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- Definition

For presentations that combine terminations, 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 or that expired.

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Weighted average price of options that were either forfeited or expired.

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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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- Definition

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

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
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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.

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Weighted average price at which grantees can acquire the shares reserved for issuance under the stock option plan.

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- Definition

Amount by which current fair value of underlying stock exceeds exercise price of fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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- Definition

Number of fully vested and expected to vest options outstanding that can be converted into shares under option plan. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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Weighted-average exercise price, at which grantee can acquire shares reserved for issuance, for fully vested and expected to vest options outstanding. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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- Definition

Weighted average price at which option holders acquired shares when converting their stock options into shares.

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Weighted average per share amount at which grantees can acquire shares of common stock by exercise of options.

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Amount of difference between fair value of the underlying shares reserved for issuance and exercise price of vested portions of options outstanding and currently exercisable.

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Weighted average remaining contractual term for vested portions of options outstanding and currently exercisable or convertible, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for option awards outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Weighted average remaining contractual term for fully vested and expected to vest options outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, unvested options for which requisite service period has not been rendered but that are expected to vest based on achievement of performance condition, if forfeitures are recognized when they occur.

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

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-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
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