Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Net Loss before Income Tax Based on Jurisdictions

Our net loss before income tax was subject to tax in the following jurisdictions for the following periods (in thousands):

	Year Ended December 31,
	2017			2016			2015
United States	$	(52,066	)	$	(40,929	)	$	(51,779	)
Foreign	(12		)	(40		)	(244		)
	$	(52,078	)	$	(40,969	)	$	(52,023	)

Income Tax Rate Reconciliation

Our rate reconciliation consists of the following:

	Year Ended December 31,
	2017		2016		2015
Federal statutory rate	35.0	%	35.0	%	35.0	%
State tax (net of federal benefit)	0.0	%	0.1	%	5.8	%
Effects of U.S. tax rate change	(47.6	)%	0.0	%	0.0	%
Federal and state tax credits	46.8	%	2.9	%	3.0	%
Uncertain tax positions	(5.3	)%	(16.0	)%	0.0	%
Stock options	(1.4	)%	(1.5	)%	(1.0	)%
Other	(0.2	)%	(2.5	)%	0.5	%
Change in valuation allowance	(27.3	)%	(18.0	)%	(43.3	)%
Effective tax rate	0.0	%	0.0	%	0.0	%

Deferred Tax Assets

	December 31,
	2017			2016
	(in thousands)
Deferred tax assets:
Federal and state tax credits	$	43,600		$	6,056
Net operating loss carryforwards	35,903			58,722
Stock-based compensation	2,371			2,760
Foreign net operating loss carryforwards	169			256
Other, net	1,809			1,816
Total deferred tax assets	83,852			69,610
Less valuation allowance	(83,852		)	(69,610		)
	$	—		$	—

Unrecognized Tax Benefit

The following table summarizes the activity related to our uncertain tax positions (in thousands):

	Year Ended December 31,
	2017		2016
Balance at beginning of year	$	16,095	$	1,422
Additions based on tax positions related to the current year	5,134		4,408
Changes for prior period tax positions	2,041		10,265
Balance at end of year	$	23,270	$	16,095