Income Taxes (Tables)

12 Months Ended

Dec. 31, 2019

Income Tax Disclosure [Abstract]

Loss Before Income Taxes

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

	2019			2018			2017
Domestic	$	(41,720	)	$	(78,124	)	$	(44,324	)
Foreign	1,293			973			966
Loss before income taxes	$	(40,427	)	$	(77,151	)	$	(43,358	)

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):

	2019		2018		2017
Current:
Domestic	$	—	$	—	$	—
Foreign	269		249		204
Total provision for income taxes	$	269	$	249	$	204

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):

	2019			2018			2017
Income tax provision at federal statutory rate	$	(8,490	)	$	(16,202	)	$	(15,076	)
Tax on repatriated foreign earnings and other nondeductible items	403			195			179
Section 162(m) limitations	1,438			—			—
Change in tax credits	(3,738		)	(2,148		)	(2,361		)
Change in valuation allowance	17,842			19,935			(19,792		)
Changes in federal and state tax rates	(4,058		)	—			37,690
Stock option exercise (windfall) shortfall	(1,763		)	257			—
Foreign tax and other	(1,365		)	(1,788		)	(436		)
Total provision for income taxes	$	269		$	249		$	204

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):

	2019			2018
Net operating loss carryforwards	$	73,310		$	63,442
Research and development tax credit carryforwards	12,636			8,491
Foreign tax credit carryforwards	633			613
Stock-based compensation	9,680			7,703
Other	10,179			8,347
Total deferred tax assets	106,438			88,596
Less: Valuation allowance	(106,438		)	(88,596		)
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):

	2019		2018		2017
Balance at beginning of year	$	88,596	$	68,661	$	88,453
Charged to costs and expenses	13,784		19,935		17,898
Impact of change in tax rate	4,058		—		(37,690		)
Balance at end of year	$	106,438	$	88,596	$	68,661

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):

	2019		2018		2017
Unrecognized tax benefits at beginning of year	$	2,830	$	2,168	$	1,524
Additions based on current year tax positions	1,382		662		644
Unrecognized tax benefits at end of year	$	4,212	$	2,830	$	2,168