Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Loss Before Income Taxes

Loss before income taxes for the years ended December 31 consisted of the following (in thousands):

	2017			2016			2015
Domestic	$	(44,324	)	$	(47,562	)	$	(46,065	)
Foreign	966			589			650
Loss before income taxes	$	(43,358	)	$	(46,973	)	$	(45,415	)

Significant Components of our Provision for Income Taxes

Significant components of our provision for income taxes for the years ended December 31 are as follows (in thousands):

	2017		2016		2015
Current:
Domestic	$	—	$	—	$	—
Foreign	204		116		166
Total provision for income taxes	$	204	$	116	$	166

Income Tax Expense (Benefit) Differed from Amounts Computed by Applying Statutory Federal Income Tax Rate

A reconciliation of the federal statutory income tax rate to the effective income tax rate for the years ended December 31 are as follows (in thousands):

	2017			2016			2015
Income tax provision at statutory rate	$	(15,076	)	$	(16,010	)	$	(15,662	)
Tax on repatriated foreign earnings and other nondeductible items	179			135			401
Change in tax credits	(2,361		)	(1,449		)	(792		)
Change in valuation allowance	(19,792		)	17,824			16,706
Change in tax rate	37,690			—			—
Foreign tax and other	(436		)	(384		)	(487		)
Total provision for income taxes	$	204		$	116		$	166

Effect of Temporary Differences and Carryforwards

The effect of temporary differences and carryforwards that give rise to deferred tax assets for the years ended December 31 were as follows (in thousands):

	2017			2016
Net operating loss carryforwards	$	49,662		$	70,694
Research and development tax credit carryforwards	6,505			4,572
Foreign tax credit carryforwards	448			—
Stock-based compensation	5,664			5,360
Other	6,382			7,827
Total deferred tax assets	68,661			88,453
Less: Valuation allowance	(68,661		)	(88,453		)
Net deferred tax assets	$	—		$	—

Summary of Changes in Deferred Tax Asset Valuation Allowance

The table below summarizes changes in the deferred tax asset valuation allowance for the years ended December 31 (in thousands):

	2017			2016		2015
Balance at beginning of year	$	88,453		$	70,629	$	53,923
Charged to costs and expenses	17,898			17,824		16,706
Impact of change in tax rate	(37,690		)	—		—
Balance at end of year	$	68,661		$	88,453	$	70,629

Total Balance of Unrecognized Gross Tax Benefits Resulting from R&D Credits Claimed

The total balance of unrecognized gross tax benefits for the years ended December 31, resulting from research and development tax credits claimed on the Company’s annual tax return was as follows (in thousands):

	2017		2016		2015
Unrecognized tax benefits at beginning of year	$	1,524	$	1,041	$	777
Additions based on current year tax positions	644		483		264
Unrecognized tax benefits at end of year	$	2,168	$	1,524	$	1,041