Leases - General Information (Details) $ in Millions	Sep. 19, 2018 USD ($) ft²	Apr. 28, 2017 USD ($) ft²	Feb. 12, 2015 USD ($) ft²	Mar. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Sep. 18, 2018 ft²	Mar. 31, 2018 USD ($)
Leases
Restricted cash				$ 5.2	$ 5.7
Corporate Headquarters Lease
Leases
Area leased (in square feet) | ft²			38,500
Extension period of lease term			5 years
Lessee, Operating Lease, Existence of Option to Extend			true
Initial lease commitments			$ 17.8
Initial lease term			7 years
Base annual rent, initial			$ 2.3
Base annual rent, maximum			2.8
Allowance for leasehold improvements			$ 4.3
Security deposit				0.9
Total commitment in sublease				1.9			$ 8.2
Sublease payable							6.9
Additional sublease payments on share of profits							$ 0.7
Office and Laboratory Space in Cambridge Massachusetts, 45 Sidney Street
Leases
Area leased (in square feet) | ft²	139,216	99,833				99,833
Extension period of lease term		5 years
Lessee, Operating Lease, Existence of Option to Extend		true
Base annual rent, initial		$ 7.7
Base annual rent, maximum		10.6
Allowance for leasehold improvements		$ 14.2
Security deposit				3.0
Office and Laboratory Space in Cambridge, Massachusetts, 45 Sidney Street, Expansion Premises
Leases
Base annual rent, initial	$ 3.2
Base annual rent, maximum	4.2
Allowance for leasehold improvements	$ 3.2
Security deposit				$ 0.8