Research Funding (Details) - USD ($) $ in Millions	1 Months Ended			12 Months Ended
	Mar. 31, 2018	Dec. 31, 2017	May 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Feb. 28, 2017
Deferred Revenue Arrangement [Line Items]
Grants receivable	$ 8.6					$ 12.2
Funding of grant award			$ 5.7
Milestone payment received on grant award		$ 1.9
Progress payment received on grant award	$ 0.3
Amortization of accrued grant funding to offset expense				$ 2.6	$ 1.3
Accrued Liabilities
Deferred Revenue Arrangement [Line Items]
Accrued grant funding				2.6
Other Long-term Liabilities
Deferred Revenue Arrangement [Line Items]
Accrued grant funding				$ 1.5