Income Taxes (Tables)

12 Months Ended

Dec. 31, 2016

Income Tax Disclosure [Abstract]

Schedule of reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations

A reconciliation of income taxes computed using the U.S. federal statutory rate to that reflected in operations follows (in thousands):

	Year Ended December 31,
	2016			2015			2014
Income tax benefit using U.S. federal statutory rate	$	(62,141	)	$	(34,391	)	$	(21,243	)
State income taxes, net of federal benefit	(5,236		)	(4,434		)	(2,494		)
Stock-based compensation	1,585			752			149
Research and development tax credits	(2,794		)	(1,469		)	(499		)
Change in the valuation allowance	48,096			39,291			23,186
Permanent items	53			26			910
Other	1,371			225			(9		)
Expiring NOLs and credits - 382 Limitation	$	19,066		—			—
Income tax expense	$	—		$	—		$	—

Schedule of principal components of the Company's deferred tax assets

The principal components of the Company's deferred tax assets are as follows (in thousands):

	December 31,
	2016			2015
Non-current assets:
NOL carryforwards	$	193,436		$	154,239
Capitalized research and development	1,970			263
Research and development credits	4,525			6,313
Depreciation and amortization	(173		)	(119		)
Accrued expenses	3,109			1,073
Stock-based compensation	14,903			7,753
Other	55			29
Gross non-current deferred tax assets	217,825			169,551
Valuation allowance	(217,825		)	(169,551		)
Net non-current deferred tax assets	$	—		$	—