Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Components of Company's Loss Before Income Taxes

The components of the Company’s loss before income taxes were as follows (in thousands):

	Year Ended December 31,
	2017		2016		2015
Domestic	$	250,917	$	192,287	$	145,618
Foreign		67,421		53,552		—
Total loss before income taxes	$	318,338	$	245,839	$	145,618

Components of Company's Income Tax Provision

The components of the Company’s income tax provision were as follows (in thousands):

	Year Ended December 31,
	2017			2016		2015
Current provision for income taxes:
Federal	$	—		$	—	$	—
State		5			—		—
International		42			35		—
Total current tax provision		47			35		—
Deferred tax benefit:
Federal		(16,243	)		—		—
State		(3	)		—		—
International		—			—		—
Total deferred tax benefit		(16,246	)		—		—
Total (benefit from) provision for income     taxes	$	(16,199	)	$	35	$	—

Effective Tax Rate of Provision for Income Taxes from Federal Statutory Rate

The effective tax rate of our provision for income taxes differs from the federal statutory rate as follows:

	Year Ended December 31,
	2017				2016				2015
Federal statutory income tax rate		34.0		%		34.0		%		34.0		%
State income taxes, net of federal benefit		—				1.3				7.5
Federal tax credits		9.0				13.7				5.9
Other		(0.9	)			(0.3	)			0.8
Nondeductible permanent items		—				—				—
Stock-based compensation		(0.6	)			(1.4	)			(1.3	)
Uncertain tax positions		(1.8	)			2.0				(7.7	)
Change in valuation allowance		(32.5	)			(41.9	)			(39.2	)
Foreign rate differential		(7.2	)			(7.4	)			—
Change in federal tax rate		5.1				—				—
Provision for income taxes		5.1				—				—

Schedule of Tax Effect of Temporary Differences to Significant Portions of Deferred Tax Assets

The tax effect of temporary differences that give rise to significant portions of the deferred tax assets is presented below (in thousands):

	Year Ended December 31,
	2017			2016
Deferred tax assets:
Loss carryforwards	$	154,949		$	102,960
Tax credits		119,542			78,324
Stock options		21,336			16,084
Accruals and reserves		5,939			10,039
Fixed assets and intangibles		1,194			3,014
Other		1,286			1,950
Gross deferred tax assets		304,246			212,371
Valuation allowance		(304,246	)		(212,371	)
Total deferred tax assets		—			—
Deferred tax liabilities:
In-process research and development		(31,166	)		—
Net deferred tax assets (liabilities)	$	(31,166	)	$	—

Unrecognized Tax Benefits

A reconciliation of the Company’s unrecognized tax benefits for the years ended December 31, 2017, 2016, and 2015 is as follows (in thousands):

	December 31,
	2017		2016			2015
Balance at beginning of year	$	13,505	$	24,010		$	7,275
Additions based on tax positions related to current    year		9,338		6,777			15,628
Additions for tax positions of prior years		5,534		877			5,505
Reductions for tax positions of prior years		—		(18,159	)		(4,398	)
Balance at end of year	$	28,377	$	13,505		$	24,010