Income Taxes (Tables)

12 Months Ended

Dec. 31, 2017

Income Tax Disclosure [Abstract]

Summary of Deferred Tax Assets

The principal components of the Company’s deferred tax assets are as follows:

	As of December 31,
	2017			2016
Deferred tax assets:
Net operating loss carryforwards	$	111,819		$	102,963
Deferred revenue		1,593			2,240
Accrued bonus		3,609			2,963
Deferred rent		3,400			4,137
Nonaccrual receivables		18,510			8,437
Other		3,127			3,509
Research and development credits		17,067			12,512
Gross deferred tax assets		159,125			136,761
Deferred tax liability		(40	)		(3,124	)
Valuation allowance		(159,085	)		(133,637	)
Net deferred tax assets	$	—		$	—

Reconciliation of Income Tax Expense at Federal Tax rate

A reconciliation of the income tax expense at the federal statutory tax rate to the Company’s effective income tax rate follows:

	Year Ended December 31,
	2017			2016			2015
Statutory tax rate		34.0	%		34.0	%		34.0	%
State taxes, net of federal benefit		5.6	%		3.3	%		4.8	%
Permanent differences		4.1	%		(0.9	)%		(1.0	)%
Research and development credits		2.2	%		2.6	%		4.5	%
Federal deferred rate change		(39.2	)%		0.0	%		0.0	%
Other		(0.1	)%		(2.4	)%		(0.6	)%
Change in valuation allowance		(6.6	)%		(36.6	)%		(41.7	)%
Effective tax rate		0.0	%		0.0	%		0.0	%