Income Taxes -Schedule of Significant Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Deferred tax assets:
Accrued expenses	$ 4,305	$ 2,170
Deferred revenue	32,093	12,157
Intangible assets	509	449
Restricted stock		45
Non-qualified stock options	3,129	2,963
Research and development credits	25,322	10,510
Net operating loss carryforward	42,335	84,372
Other	600	326
Total deferred tax assets	108,293	112,992
Less valuation allowance	(108,112)	(112,986)
Total deferred tax assets, net of valuation allowance	181	6
Deferred tax liabilities:
Fixed assets	(181)	(6)
Total deferred tax liabilities	(181)	(6)
Net deferred tax asset	$ 0	$ 0