STOCK BASED COMPENSATION - Plans (Details) - shares	12 Months Ended
Dec. 31, 2018	Sep. 30, 2019	Dec. 31, 2017
Stock-based compensation
Options outstanding (in shares)	1,202,979	3,767,571	262,427
2016 Plan
Stock-based compensation
Shares reserved (in shares)	2,966,241		1,345,469
Expiration period	10 years
Vesting period	3 years
Number of shares available for future issuance	1,708,179		1,083,041

Aggregate number of common shares reserved for future issuance.

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-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
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Period over which grantee's right to exercise award under share-based payment arrangement is no longer contingent on satisfaction of service or performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days. Includes, but is not limited to, combination of market, performance or service condition.

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-Publisher FASB
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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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- Definition

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

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- Definition

Period from grant date that an equity-based award expires, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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