Income Taxes - Summary of Significant Components of Company's Net Deferred Income Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016
Net deferred tax assets:
Net operating loss carryforwards	$ 58,172	$ 39,188
Stock-based compensation	13,681	11,845
U.S. tax credit carryforwards	4,182	4,566
Envisia asset acquisition	7,107	0
Other assets	242	1,138
Valuation allowance	(83,384)	(56,737)
Total net deferred income taxes	$ 0	$ 0