Commitment and Contingencies - Additional Information (Details)	1 Months Ended		3 Months Ended
	Dec. 31, 2017 USD ($) ft² Phase	Sep. 30, 2016	Mar. 31, 2019 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2018 USD ($)
Commitment And Contingencies [Line Items]
Deferred rent incentives			$ 9,200,000
Rent expense, net			200,000	$ 900,000
Letters Of Credit And Secured By Restricted Cash
Commitment And Contingencies [Line Items]
Security deposit			1,300,000		$ 1,800,000
Maximum
Commitment And Contingencies [Line Items]
Tenant improvement allowance			10,900,000
Bedford, Massachusetts
Commitment And Contingencies [Line Items]
Operating lease beginning year and month		2016-11
Operating lease expiration year and month	2027-02	2021-10
Operating lease agreements additional term		3 years
Sublease aggregate base rent obligation			$ 2,700,000
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
Rent Start Date Year And Month	2018-09
Bedford, Massachusetts | Phase Two
Commitment And Contingencies [Line Items]
Office space leased | ft²	21,000
Rent Start Date Year And Month	2019-03