Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Taxes

Schedule of effective tax rates differing from U.S. federal statutory rate

	Year Ended December 31,
	2018				2017				2016
	(in thousands, except percentages)
U.S. federal taxes (benefit) at statutory rate	$	(26,800)	(21.00)	%	$	(46,206)	(34.00)	%	$	(32,277)	(34.00)	%
State tax expense		(4,468)	(3.50)	%		(6,559)	(4.83)	%		(2,842)	(2.99)	%
Research and development credits		(1,164)	(0.91)	%		(1,149)	(0.85)	%		(1,449)	(1.53)	%
Stock-based compensation		(3,148)	(2.47)	%		(5,036)	(3.71)	%		1,275	1.34	%
Change in federal tax rate		—	0.00	%		44,701	32.89	%		—	0.00	%
Mark to market fair value adjustments		865	0.68	%		(390)	(0.29)	%		49	0.05	%
Nondeductible settlement for claims		1	0.00	%		3,873	2.85	%		—	0.00	%
Other nondeductible items		885	0.69	%		1,009	0.74	%		933	0.99	%
Change in valuation allowance		34,150	26.76	%		10,211	7.51	%		34,453	36.29	%
Provision for income taxes	$	321	0.25	%	$	454	0.31	%	$	142	0.15	%

Schedule of tax effects of temporary differences that give rise to significant portions of the deferred tax assets

	December 31,
	2018		2017
	(in thousands)
Deferred tax assets:
Net operating loss carryforwards	$	95,321	$	77,356
Research and development tax credit carryforwards		15,632		12,675
Reserves and accruals		7,922		3,404
Stock-based compensation		4,076		2,929
Total deferred tax assets before valuation allowance		122,951		96,364
Less: valuation allowance		(122,441)		(95,817)
		510		547
Deferred tax liabilities:
Property and equipment		(510)		(547)
Net deferred tax assets	$	—	$	—

Schedule of reconciliation of the beginning and ending amount of gross unrecognized tax benefits

	Year Ended December 31,
	2018		2017		2016
	(in thousands)
Balance at beginning of year	$	5,945	$	4,293	$	2,405
Additions based on tax positions related to the current year		1,416		1,651		1,836
Additions for tax positions of prior years		1		1		52
Balance at end of year	$	7,362	$	5,945	$	4,293