Capitalization and Warrant Instruments - Common Stock Shares Reserved for Future Issuance (Details) - shares	Dec. 31, 2018	Dec. 31, 2017
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares reserved for future issuance	34,596,108	17,452,125
Stock-based compensation plans
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares reserved for future issuance	16,096,108	16,952,125
Healios Warrants to purchase common stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares reserved for future issuance	18,500,000	0
Shares issuable upon patent milestone
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Common stock shares reserved for future issuance	0	500,000