Income Taxes - Components of Deferred Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred Tax Assets:
Accumulated net operating losses (tax effected)	$ 70,747	$ 60,544
CIRM funding	336	1,033
Right of use liability	205	5
Share-based compensation	3,584	6,390
Intangibles	141	199
Charitable contributions	0	9
Accumulated depreciation	9	9
Accrued payroll	121	117
Other	737	525
Deferred tax assets	75,880	68,831
Deferred Tax Liabilities:
Right of use asset	190	0
Deferred tax liabilities	190	0
Net deferred tax asset, before valuation allowance	75,690	68,831
Valuation allowance	(75,690)	(68,831)
Net deferred tax asset	$ 0	$ 0