Income taxes - Narrative (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022
Components Of Income Tax Expense Benefit [Line Items]
Income tax expense (benefit)	$ (126,000)	$ 117,000
Effective income tax rate	0.10%	(0.10%)
Approximately valuation allowance increased	$ 73,300,000
Operating loss carryforwards, without expiration dates	2,000,000,000.0
Operating loss carryforwards, with expiration dates	700,000,000
Capital loss carryforwards	4,200,000	$ 4,200,000
Income tax examination, penalties and interest accrued	$ 0	0
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	$ 2,700,000,000	2,700,000,000
U.S. | Research Tax Credit Carryforward
Components Of Income Tax Expense Benefit [Line Items]
Tax credit carryforward amount	299,500,000	285,600,000
State and Local Jurisdiction
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	2,500,000,000	2,500,000,000
State and Local Jurisdiction | Research Tax Credit Carryforward
Components Of Income Tax Expense Benefit [Line Items]
Tax credit carryforward amount	$ 18,300,000	$ 18,500,000