Leases - Additional Information (Detail)			1 Months Ended	6 Months Ended
	Jun. 03, 2016 USD ($)	Sep. 21, 2015 USD ($) ft² $ / ft²	Apr. 30, 2019 USD ($) ft² $ / ft²	Jun. 30, 2019 USD ($)	Jan. 01, 2019 USD ($)
Lessee Lease Description [Line Items]
Operating lease, right-of-use asset				$ 190,979,000	$ 127,300,000
Operating lease, liability				$ 194,222,000	$ 125,800,000
Building [Member]
Lessee Lease Description [Line Items]
Useful life of asset				40 years
50 Binney Street Sublease [Member]
Lessee Lease Description [Line Items]
Lease rent, annual increase percentage			3.00%
Sublease rent description				Under the terms of the 50 Binney Street Sublease, the Company will lease 267,278 square feet of office space for $99.95 per square foot, or $26.7 million per year in base rent subject to certain operating expenses, taxes and annual rent increases of approximately 3%. The lease will commence when the space is available for use by the Company, which is anticipated to be July 1, 2021, and end on December 31, 2030, unless the Company earlier occupies the premises or other conditions specified in the 50 Binney Street Sublease occur.   The sublessor has the right to postpone the commencement date until January 1, 2022 by providing not less than nine months’ prior written notice to the Company.
Area of office space for sublease | ft²			267,278
Rate of area for sublease per square feet | $ / ft²			99.95
Sublease annual base rent			$ 26,700,000
Sublease rent payments				Payments will commence at the earlier of (i) the date which is 90 days following the commencement date and (ii) the date the Company takes occupancy of all or any portion of the premises
50 Binney Street Sublease [Member] | Cash-Collateralized Letter of Credit [Member]
Lessee Lease Description [Line Items]
Collateralized letter of credit			40,100,000
50 Binney Street Sublease [Member] | Cash-Collateralized Letter of Credit [Member] | Restricted Cash and Other Non-current Assets [Member]
Lessee Lease Description [Line Items]
Collateralized letter of credit			$ 40,100,000
Furniture Purchase Agreement [Member] | Restricted Cash and Other Non-current Assets [Member]
Lessee Lease Description [Line Items]
Upfront payment for purchase of furniture and equipment				$ 7,500,000
Manufacturing Agreement [Member]
Lessee Lease Description [Line Items]
Lease period	12 years
Milestone paid	$ 12,000,000.0
Lease payments	$ 5,100,000
Agreement termination description				The Company may terminate this agreement at any time upon payment of a one-time termination fee and up to 24 months of fixed suite and labor fees.
Early termination lease term	24 months
Manufacturing Agreement [Member] | Maximum [Member]
Lessee Lease Description [Line Items]
Additional contractual payables to milestones	$ 8,000,000.0
Lease starting on October 1, 2016 [Member] | 60 Binney Street Lease [Member]
Lessee Lease Description [Line Items]
Lease period		10 years
Lease starting date		Oct. 01, 2016
Lease building space | ft²		253,108
Annual lease rent per square foot | $ / ft²		72.50
Lease payments base annual rent		$ 18,400,000
Operating lease description				Under the terms of the 60 Binney Street Lease, starting on October 1, 2016, the Company leases approximately 253,108 square feet of office and laboratory space at $72.50 per square foot per year, or $18.4 million per year in base rent, which is subject to scheduled annual rent increases of 1.75% plus certain operating expenses and taxes.
Lease rent, annual increase percentage		1.75%
Collateralized letter of credit		$ 13,800,000
Landlord contribution for cost of construction and tenant improvements		42,400,000
Decrease in letter of credit under the terms of the lease		$ 9,200,000
Lease expiration date		Mar. 31, 2027
Lease extension terms		two successive five-year terms