Income Taxes - Narrative (Details) $ in Millions	Dec. 31, 2019 USD ($)
Income Tax Contingency [Line Items]
Unrecognized tax benefits	$ 0.2
Domestic Tax Authority
Income Tax Contingency [Line Items]
Operating loss carryforwards	181.8
Operating loss carryforwards, not subject to expiration	127.7
State and Local Jurisdiction
Income Tax Contingency [Line Items]
Operating loss carryforwards	177.4
Foreign Tax Authority
Income Tax Contingency [Line Items]
Operating loss carryforwards	64.4
Research Tax Credit Carryforward | Domestic Tax Authority
Income Tax Contingency [Line Items]
Tax credit carryforward, amount	3.1
Research Tax Credit Carryforward | State and Local Jurisdiction
Income Tax Contingency [Line Items]
Tax credit carryforward, amount	0.6
Merger, Arsanis | Domestic Tax Authority
Income Tax Contingency [Line Items]
Operating loss carryforwards	75.0
Merger, Arsanis | State and Local Jurisdiction
Income Tax Contingency [Line Items]
Operating loss carryforwards	$ 70.0