Leases (Details) $ in Thousands	3 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 ft²	Dec. 31, 2018 USD ($)
Future operating lease payments
2019 (remaining nine months)	$ 1,272
2020	1,728
2021	851
2022	47
Total undiscounted lease payments	3,898
Less: Present value adjustment	(256)
Total lease liability	3,642
Less: Current portion of lease liability	(1,532)
Total lease liability, net of current portion	2,110
Future finance lease payments
2019 (remaining nine months)	8
2020	10
Total undiscounted lease payments	18
Less: Present value adjustment	(1)
Total lease liability	17
Less: Current portion of lease liability	(10)
Total lease liability, net of current portion	7
Future lease payments
2019 (remaining nine months)	1,280
2020	1,738
2021	851
2022	47
Total undiscounted lease payments	3,916
Less: Present value adjustment	(257)
Total lease liability	3,659
Less: Current portion of lease liability	(1,542)
Total lease liability, net of current portion	$ 2,117
Minimum rental commitments
2019				$ 1,645
2020				1,701
2021				846
2022				47
Total minimum lease payments				$ 4,239
Other lease information
Operating lease weighted-average remaining lease term (years)	2 years 4 months 10 days
Finance lease weighted-average remaining lease term (years)	1 year 8 months 1 day
Operating lease weighted-average discount rate	6.00%
Finance lease weighted-average discount rate	3.50%
Operating lease costs	$ 396	$ 327
Previous Headquarters
Leases
Area of operating lease (in sq feet) | ft²			27,974
Office and Lab Space
Leases
Operating lease renewal term (in years)	5 years
Arizona manufacturing facility
Leases
Period to cancel lease upon notice	24 months