Income Taxes - Components of Deferred Income Taxes (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Components of Deferred Tax Assets and Liabilities [Abstract]
Accumulated net operating losses (tax effected)	$ 60,544	$ 60,171
CIRM funding	1,033	1,780
Deferred rent	5	3
Share-based compensation	6,390	2,656
Intangibles	199	270
Charitable contributions	9	11
Accumulated depreciation	9	22
Accrued payroll	117	682
AMT credit	0	575
Other	525	526
Deferred tax assets	68,831	66,696
Deferred tax liabilities	0	0
Net deferred tax asset, before valuation allowance	68,831	66,696
Valuation allowance	(68,831)	(66,121)
Net deferred tax asset	$ 0	$ 575