Research Funding (Details) - USD ($)	1 Months Ended				12 Months Ended
	May 31, 2019	Mar. 31, 2018	Dec. 31, 2017	May 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Feb. 28, 2017
Deferred Revenue Arrangement [Line Items]
Grants receivable		$ 8,100,000					$ 12,200,000
Funding of grant award				$ 5,700,000
Milestone payment received on grant award			$ 1,900,000
Progress payment received on grant award	$ 200,000	$ 300,000
CIRM upfront funding - current					$ 1,600,000	$ 2,583,000
Amortization of accrued grant funding to offset expense					2,700,000	$ 2,600,000
Accrued Liabilities
Deferred Revenue Arrangement [Line Items]
CIRM upfront funding - current					$ 1,600,000