Income Taxes - Summary of Significant Components of Company's Net Deferred Income Tax Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Net deferred tax assets:
Net operating loss carryforwards	$ 112,375	$ 58,172
Stock-based compensation	17,734	13,681
U.S. tax credit carryforwards	5,996	4,182
Envisia asset acquisition	5,888	7,107
Other assets	2,857	591
Other liabilities	(1,535)	(349)
Valuation allowance	(143,315)	(83,384)
Total net deferred income taxes	$ 0	$ 0