Commitments (Details) $ in Millions			3 Months Ended		9 Months Ended
	Oct. 01, 2017 USD ($) period	Feb. 12, 2015 USD ($) ft²	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Apr. 28, 2017 USD ($) ft²
Operating Leases
Rent expense			$ 2.4	$ 0.5	$ 3.9	$ 1.4
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
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
Lease term	146 months
Base annual rent, initial	$ 7.7
Base annual rent, maximum	$ 10.6
Tenant improvement allowance							$ 14.2
Balance from the leasehold improvement incentives, included in lease incentive obligation			$ 7.0		$ 7.0
Security deposit included in restricted cash							$ 3.5