Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets
Net operating loss carryforwards and credits	$ 5,302	$ 1,694
Share-based compensation expense	3,548	2,203
Property, plant and equipment	1,399	3
Other	51	11
Less: valuation allowance	(10,300)	(3,911)
Total deferred tax assets	0	0
Deferred tax liabilities
Total deferred tax liabilities	0	0
Net deferred tax assets (liabilities)	0	$ 0
Net operating loss carryforwards	25,200
Net operating loss carryforward subject to expiration	$ 7,800