Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Schedule of Components of Income Tax Expense (Benefit)

The following table summarizes the components of our income tax (benefit) expense (in thousands):

	Year ended December 31,
	2017			2016			2015
Current:
Federal	$	(143	)	$	—		$	—
State	1			1			1
	(142		)	1			1
Deferred:
Federal	(55		)	(5		)	(17		)
State	—			—			(2		)
	(55		)	(5		)	(19		)
Income tax (benefit)	$	(197	)	$	(4	)	$	(18	)

Schedule of Effective Income Tax Rate Reconciliation

The following is a reconciliation of the expected statutory federal income tax provision to our actual income tax provision (in thousands):

	Year ended December 31,
	2017			2016			2015
Expected income tax benefit at federal statutory tax rate	$	(24,515	)	$	(27,826	)	$	(18,960	)
State income taxes, net of federal benefit	1			(1,152		)	(1,580		)
Tax credits	(5,151		)	(5,447		)	(5,039		)
Change in fair value of convertible note payable	—			—			616
Change in valuation allowance	(8,705		)	31,990			23,216
Prior year true-up	1			(1,320		)	110
Stock compensation	1,135			2,458			1,161
Reserve for uncertain tax positions	311			1,314			254
Tax Cuts and Jobs Act rate change	36,249			—			—
Other	477			(21		)	204
Income tax (benefit)	$	(197	)	$	(4	)	$	(18	)

Schedule of Deferred Tax Assets and Liabilities

The following table summarizes the significant components of our deferred tax assets and liabilities (in thousands):

	December 31,
	2017			2016
Deferred tax assets:
Net operating loss carryovers	$	66,549		$	70,739
Research and development and other tax credits	29,702			22,613
Deferred revenue	419			735
Stock compensation expense	4,238			5,664
Other	2,196			4,679
Total deferred tax assets	103,104			104,430
Total deferred tax liabilities	(408		)	(799		)
Net deferred tax asset	102,696			103,631
Valuation allowance	(102,641		)	(103,631		)
Net deferred tax asset	$	55		$	—

Summary of Income Tax Contingencies

The following table summarizes the changes in the amount of our unrecognized tax benefits (in thousands):

	Year Ended December 31,
	2017			2016		2015
Beginning balance of unrecognized tax benefits	$	5,632		$	2,298	$	1,853
(Decrease) increase for prior year tax positions	(2,466		)	1,991		(250		)
Increase for current year tax positions	1,255			1,343		695
Total	$	4,421		$	5,632	$	2,298