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.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iii)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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- Definition

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.

+ References

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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
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
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-Section 50
-Paragraph 2
-Subparagraph (c)(1)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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- Definition

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

+ References

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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-URI http://asc.fasb.org/extlink&oid=109197908&loc=d3e5070-113901

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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.

+ References

No definition available.

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- Definition

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.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
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Stock Option Plan - Information by Exercise Price (Details) - $ / shares	3 Months Ended	12 Months Ended
	Mar. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)	4,138,635	4,364,377	4,308,890	3,478,346
Weighted Average Remaining Contractual Term (in Years)	7 years 3 months 3 days	7 years 4 months 9 days	7 years 9 months 21 days	7 years 8 months 15 days
Weighted Average Exercise Price (in USD per share)	$ 4.00	$ 3.79	$ 3.09	$ 1.46
Number Exercisable (in shares)	2,451,044	2,459,116
Weighted Average Exercise Price, vested and exercisable (in USD per share)	$ 2.70	$ 2.35
$0.68
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		101,848
Weighted Average Remaining Contractual Term (in Years)		1 year 7 months 2 days
Weighted Average Exercise Price (in USD per share)		$ 0.68
Number Exercisable (in shares)		101,848
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 0.68
$1.35
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		107,938
Weighted Average Remaining Contractual Term (in Years)		2 years 9 months
Weighted Average Exercise Price (in USD per share)		$ 1.35
Number Exercisable (in shares)		107,938
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 1.35
$1.38
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		304,396
Weighted Average Remaining Contractual Term (in Years)		3 years 11 months 15 days
Weighted Average Exercise Price (in USD per share)		$ 1.38
Number Exercisable (in shares)		304,396
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 1.38
$1.46
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		385,070
Weighted Average Remaining Contractual Term (in Years)		6 years 2 months 15 days
Weighted Average Exercise Price (in USD per share)		$ 1.46
Number Exercisable (in shares)		360,470
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 1.46
$1.57
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		97,311
Weighted Average Remaining Contractual Term (in Years)		8 years 25 days
Weighted Average Exercise Price (in USD per share)		$ 1.57
Number Exercisable (in shares)		42,013
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 1.57
$1.60
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		1,463,632
Weighted Average Remaining Contractual Term (in Years)		6 years 9 months 25 days
Weighted Average Exercise Price (in USD per share)		$ 1.60
Number Exercisable (in shares)		1,128,569
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 1.60
$3.16
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		462,764
Weighted Average Remaining Contractual Term (in Years)		8 years 11 months 12 days
Weighted Average Exercise Price (in USD per share)		$ 3.16
Number Exercisable (in shares)		162,549
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 3.16
$4.73
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		332,385
Weighted Average Remaining Contractual Term (in Years)		8 years 11 months 1 day
Weighted Average Exercise Price (in USD per share)		$ 4.73
Number Exercisable (in shares)		86,506
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 4.73
$6.11
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		552,462
Weighted Average Remaining Contractual Term (in Years)		9 years 3 months 25 days
Weighted Average Exercise Price (in USD per share)		$ 6.11
Number Exercisable (in shares)		29,848
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 6.11
$8.27
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		78,941
Weighted Average Remaining Contractual Term (in Years)		9 years 11 months 4 days
Weighted Average Exercise Price (in USD per share)		$ 8.27
Number Exercisable (in shares)		1,527
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 8.27
$12.15
Share-based Compensation, Shares Authorized under Stock Option Plans, Exercise Price Range [Line Items]
Options Outstanding (in shares)		477,630
Weighted Average Remaining Contractual Term (in Years)		8 years 11 months 1 day
Weighted Average Exercise Price (in USD per share)		$ 12.15
Number Exercisable (in shares)		133,452
Weighted Average Exercise Price, vested and exercisable (in USD per share)		$ 12.15