Income Taxes - Reconciliation of Effective Tax Rate to the Statutory Rate (Details) - USD ($) $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017
Amount
Income tax benefit at federal statutory rate									$ (11,457)	$ (16,037)
State income tax benefit, net of federal tax effect									(1,762)	(2,283)
Non-deductible warrants expense									2,184	1,360
Other permanent differences									70	106
Research and development tax credits									(1,058)	(426)
Other tax credits									(934)	(419)
Tax reserve reassessment									196	104
Federal tax rate change									0	10,899
Change in valuation allowance									12,856	4,956
3PI Settlement									0	1,976
Other, net									74	207
Total tax expense	$ 243	$ 118	$ 80	$ (272)	$ (320)	$ 149	$ 135	$ 479	$ 169	$ 443
Percent
Income tax benefit at federal statutory rate									21.00%	34.00%
State income tax benefit, net of federal tax effect									3.20%	4.80%
Non-deductible warrants expense									(4.00%)	(2.90%)
Other permanent differences									(0.10%)	(0.20%)
Research and development tax credits									1.90%	0.90%
Other tax credits									1.70%	0.90%
Tax reserve reassessment									(0.40%)	(0.20%)
Federal tax rate change									0.00%	(23.10%)
Change in valuation allowance									(23.50%)	(10.40%)
3PI Settlement									0.00%	(4.20%)
Other, net									(0.10%)	(0.50%)
Income tax expense									(0.30%)	(0.90%)