Accounting Policies - Instruments Excluded from Calculation of Diluted Net Loss Per Share (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	31,112,196	10,568,099	12,330,914
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	10,955,508	8,919,113	9,236,228
Restricted stock units
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	1,656,688	1,648,986	1,201,159
Warrants, including Healios Warrant
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total	18,500,000	0	1,893,527