Summary of significant accounting policies and basis of presentation - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2019 USD ($) target	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Jan. 01, 2019 USD ($)	Jan. 01, 2018 USD ($)	Jan. 01, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of operating segments | target	1
Goodwill, impaired, accumulated impairment loss	$ 0
Interest income net of amortization of premium and accretion of discount	34,800,000	$ 30,100,000	$ 9,500,000
Interest expenses	0.0	15,500,000	$ 11,400,000
Operating lease right-of-use assets	185,885,000
Total operating lease liabilities	190,987,000
Property, plant and equipment, net	151,176,000	246,622,000
Financing lease obligation, current portion	0	1,424,000
Cumulative effect of new accounting principle in period of adoption						$ 0
Deferred tax assets, gross	799,237,000	588,546,000		$ 5,300,000
Deferred tax liabilities, gross				7,100,000
Deferred tax liabilities, net	$ 0	$ 0		1,800,000
Accumulated Deficit
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of new accounting principle in period of adoption				6,564,000	$ (29,375,000)	$ (491,000)
Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use assets				184,400,000
Total operating lease liabilities				177,000,000.0
Financing lease obligation, current portion				1,400,000
Finance lease, liability, noncurrent				153,300,000
60 Binney Street lease | Accounting Standards Update 2016-02
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease right-of-use assets				127,300,000
Total operating lease liabilities				125,800,000
Property, plant and equipment, net				$ 149,300,000