Stock-Based Compensation (Details) - USD ($)	1 Months Ended	3 Months Ended	12 Months Ended			36 Months Ended
	Jan. 31, 2017	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017
Stock-Based Compensation
Shares issued under the Employee Stock Purchase Plan (in shares)			$ 656,000	$ 425,000	$ 467,000
Service-based stock options
Stock-Based Compensation
Vesting period			4 years
Stock-based compensation expense			$ 6,900,000	$ 2,500,000	$ 2,300,000
Weighted average grant-date fair value (in dollars per share)			$ 6.96	$ 1.99	$ 2.15
Weighted average assumptions used to estimate fair value of each service-based stock option grant
Expected dividend rate			0.00%	0.00%	0.00%
Options
Outstanding at the beginning of the period (in shares)			4,528,426
Granted (in shares)			1,644,160
Exercised (in shares)			(1,180,815)
Expired (in shares)			(12,862)
Forfeited (in shares)			(188,226)
Outstanding at the end of the period (in shares)		4,528,426	4,790,683	4,528,426		4,528,426
Exercisable at the end of the period (in shares)			2,292,065
Weighted Average Exercise Price
Outstanding at the beginning of the period (in dollars per share)			$ 3.77
Granted (in dollars per share)			9.64
Exercised (in dollars per share)			3.14
Expired (in dollars per share)			6.84
Forfeited or expired (in dollars per share)			5.89
Outstanding at the end of the period (in dollars per share)		$ 3.77	5.85	$ 3.77		$ 3.77
Exercisable at the end of the period (in dollars per share)			$ 5.40
Weighted Average Remaining Contractual Term
Outstanding			7 years 8 months 12 days	8 years
Aggregate Intrinsic Value
Outstanding		$ 10,776,000	$ 11,407,000	$ 10,776,000		$ 10,776,000
Exercised
Exercisable at end of period			4,840,000
Additional disclosures
Total unrecognized compensation cost			$ 7,200,000
Expected to vest (in shares)			4,481,963
Weighted average period over which unrecognized compensation is expected to be recognized			3 years 12 days
Total intrinsic value of options vested			$ 10,300,000	$ 700,000	$ 100,000
Service-based stock options | Minimum
Weighted average assumptions used to estimate fair value of each service-based stock option grant
Expected stock price volatility			82.30%	79.70%	78.70%
Risk-free interest rate			2.40%	1.39%	1.10%
Expected life (years)			5 years 3 months 18 days	5 years 6 months	5 years 6 months
Service-based stock options | Maximum
Stock-Based Compensation
Expiration period			10 years
Weighted average assumptions used to estimate fair value of each service-based stock option grant
Expected stock price volatility			88.30%	88.20%	92.20%
Risk-free interest rate			3.10%	2.30%	2.10%
Expected life (years)			6 years 3 months 18 days	6 years 3 months 18 days	6 years 3 months 18 days
Service-based stock options | Non-employee
Stock-Based Compensation
Expiration period			10 years
Vesting period			1 year
Options
Granted (in shares)			105,000
Employee stock
Stock-Based Compensation
Common stock available for issuance (in shares)			1,000,000
Common stock granted since inception (in shares)						526,020
Shares purchased during period (in shares)	18,407
Shares issued under the Employee Stock Purchase Plan (in shares)		$ 200,000
Stock-based compensation expense			$ 300,000	$ 200,000	$ 200,000
Prior Plans
Stock-Based Compensation
Awards available for future grant under the Plan (in shares)			0
Omnibus Incentive Plan 2017
Stock-Based Compensation
Awards available for future grant under the Plan (in shares)			2,983,774