Commitment and Contingencies - Additional Information (Details)	1 Months Ended		3 Months Ended		9 Months Ended
	Dec. 31, 2017 USD ($) ft² Phase	Sep. 30, 2016	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 01, 2018 USD ($)
Commitment And Contingencies [Line Items]
Deferred rent incentives			$ 5,100,000		$ 5,100,000
Rent expense			500,000	$ 200,000	2,300,000	$ 700,000
Letters Of Credit And Secured By Restricted Cash
Commitment And Contingencies [Line Items]
Security deposit	$ 1,800,000		1,800,000		1,800,000
Maximum
Commitment And Contingencies [Line Items]
Tenant improvement allowance			$ 10,900,000		$ 10,900,000
Bedford, Massachusetts
Commitment And Contingencies [Line Items]
Operating lease expiration year and month	2027-02	2021-10
Operating lease agreements additional term		3 years
Office space leased | ft²	67,000
Lessee, operating lease, lease not yet commenced, renewal term	5 years
Number of phases | Phase	2
Initial annual base rent per square foot	$ 39.50
Percentage increase in initial annual base rent per square foot.	3.00%
Bedford, Massachusetts | Phase One
Commitment And Contingencies [Line Items]
Office space leased | ft²	46,000
Bedford, Massachusetts | Phase Two
Commitment And Contingencies [Line Items]
Office space leased | ft²	21,000
Bedford, Massachusetts | Scenario, Forecast [Member]
Commitment And Contingencies [Line Items]
Sublease aggregate base rent obligation							$ 2,700,000