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Stock-based Compensation - Narrative (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		66 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 2,435	$ 2,202	$ 7,552	$ 6,325
Plan options contractual life (up to)			10 years
Award vesting period			4 years
Unrecognized compensation cost	21,214		$ 21,214
Estimated weighted-average amortization period			2 years 6 months 22 days
Service Based Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Award vesting period			4 years	4 years
Stock options to purchase shares (in shares)			2,070,173	423,892
Weighted average grant date fair value (usd per share)			$ 4.78	$ 20.35
Restricted Stock Units (RSUs)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Granted (in shares)			20,600
Restricted shares fair value at grant date (usd per share)			$ 10.20
2017 Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense	$ 91	$ 84	$ 319	$ 363
Shares issued (in shares)			77,909	53,137	31,892
Inventory
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			$ 62
Prepaid expenses and other assets
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock-based compensation expense			$ 29