Commitments - General Information (Details) $ in Millions			3 Months Ended	6 Months Ended
	Oct. 01, 2017 USD ($) ft² period	Feb. 12, 2015 USD ($) ft²	Mar. 31, 2018 USD ($)	Jun. 30, 2018 USD ($)
Operating Leases
Total commitment under the operating lease				$ 6.9
Corporate Headquarters Lease
Operating Leases
Area leased (in square feet) | ft²		38,500
Extension period of lease term		5 years
Total commitment under the operating lease		$ 17.8
Lease term		7 years
Base annual rent, initial		$ 2.3
Base annual rent, maximum		2.8
Allowance for leasehold improvements		4.3
Security deposit included in restricted cash		$ 1.3
Total commitment in sublease			$ 8.2
Sublease term			32 months
Sublease payable				$ 0.7
Office and Laboratory Space in Cambridge Massachusetts, 45 Sidney Street
Operating Leases
Area leased (in square feet) | ft²	99,833
Number of extensions | period	2
Extension period of lease term	5 years
Base annual rent, initial	$ 7.7
Base annual rent, maximum	10.6
Security deposit included in restricted cash	3.5
Tenant improvement allowance	$ 14.2