Leases - Additional Information (Detail)			3 Months Ended
	Jun. 03, 2016 USD ($)	Sep. 21, 2015 USD ($) ft² $ / ft²	Mar. 31, 2019 USD ($)	Jan. 01, 2019 USD ($)
Lessee Lease Description [Line Items]
Operating lease, right-of-use asset			$ 184,618,000	$ 127,300,000
Operating lease, liability			185,766,000	$ 125,800,000
Legally binding minimum lease payments for leases executed but not yet commenced			$ 18,000,000
Building [Member]
Lessee Lease Description [Line Items]
Useful life of asset			40 years
Manufacturing Agreement [Member]
Lessee Lease Description [Line Items]
Lease period	12 years
Milestone paid	$ 12,000,000
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
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