Income taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Components Of Income Tax Expense Benefit [Line Items]
Income tax expense (benefit)	$ 686,000	$ (545,000)	$ 187,000
Effective income tax rate	(0.10%)	0.10%	0.00%
Approximately valuation allowance increased	$ 138,300,000
Operating loss carryforwards, without expiration dates	1,320,000,000
Operating loss carryforwards, with expiration dates	711,000,000.0
Cumulative-effect adjustment to retained earnings	546,098,000	$ 439,839,000
Stockholders' equity	1,355,056,000	1,284,993,000	$ 1,885,070,000	$ 1,623,432,000
Income tax examination, penalties and interest accrued	0	0	0
Cumulative Effect, Period of Adoption, Adjustment
Components Of Income Tax Expense Benefit [Line Items]
Stockholders' equity			6,564,000	(29,375,000)
Accumulated Deficit
Components Of Income Tax Expense Benefit [Line Items]
Stockholders' equity	$ (2,900,547,000)	(2,281,852,000)	(1,498,808,000)	(913,808,000)
Accumulated Deficit | Cumulative Effect, Period of Adoption, Adjustment
Components Of Income Tax Expense Benefit [Line Items]
Stockholders' equity			6,564,000	$ (29,375,000)
Research Tax Credit Carryforward
Components Of Income Tax Expense Benefit [Line Items]
Limitations on use of net operating losses and tax credit carryforwards, period	3 years
Limitations on use of net operating losses and tax credit carryforwards, percentage	50.00%
U.S.
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	$ 2,030,000,000.00	1,620,000,000	1,100,000,000
U.S. | Research Tax Credit Carryforward
Components Of Income Tax Expense Benefit [Line Items]
Tax credit carryforward amount	235,300,000	203,100,000	156,200,000
State and Local Jurisdiction
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	1,890,000,000	1,560,000,000	1,080,000,000.00
State and Local Jurisdiction | Research Tax Credit Carryforward
Components Of Income Tax Expense Benefit [Line Items]
Tax credit carryforward amount	$ 14,500,000	$ 13,600,000	$ 14,300,000