Research Funding (Details) - USD ($) $ in Thousands	1 Months Ended			3 Months Ended		9 Months Ended
	May 31, 2019	Mar. 31, 2018	May 31, 2017	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Feb. 28, 2017
Deferred Revenue Arrangement [Line Items]
Grants awarded		$ 8,600							$ 12,200
Funding of grant award			$ 5,700
Milestone payment received on grant award						$ 1,900
Progress payment received on grant award	$ 200	$ 300
Accrued grant funding				$ 2,286		2,286		$ 2,583
Research and Development [Member]
Deferred Revenue Arrangement [Line Items]
Amortization of accrued grant funding to offset expense				700	$ 600	2,000	$ 1,900
Accrued Liabilities [Member]
Deferred Revenue Arrangement [Line Items]
Accrued grant funding				$ 2,300		$ 2,300