Income taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019	Jan. 01, 2018	Jan. 01, 2017	Dec. 31, 2016
Components Of Income Tax Expense Benefit [Line Items]
Income tax expense (benefit)	$ (545,000)	$ 187,000	$ 210,000
Effective Income Tax Rate Reconciliation, Percent	0.10%	0.00%	(0.10%)
Approximately valuation allowance increased	$ (201,100,000)
Operating loss carryforwards, without expiration dates	913,800,000
Operating loss carryforwards, with expiration dates	711,000,000.0
Cumulative effect of new accounting principle in period of adoption						$ 0
Cumulative-effect adjustment to retained earnings	439,839,000	$ 298,701,000
Income tax examination, penalties and interest accrued	$ 0	0	$ 0
Accumulated Deficit
Components Of Income Tax Expense Benefit [Line Items]
Cumulative effect of new accounting principle in period of adoption				$ 6,564,000	$ (29,375,000)	(491,000)
ASU 2016-09
Components Of Income Tax Expense Benefit [Line Items]
Cumulative-effect adjustment to retained earnings						76,700,000
ASU 2016-09 | Accumulated Deficit
Components Of Income Tax Expense Benefit [Line Items]
Cumulative effect of new accounting principle in period of adoption						$ 76,700,000
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	$ 1,620,000,000	1,100,000,000	716,100,000
U.S. | Research Tax Credit Carryforward
Components Of Income Tax Expense Benefit [Line Items]
Tax credit carryforward amount	203,100,000	156,200,000	124,100,000
State and Local Jurisdiction
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards	1,560,000,000	1,080,000,000.00	692,900,000
State and Local Jurisdiction | Excess Equity Based Compensation Tax Deductions
Components Of Income Tax Expense Benefit [Line Items]
Operating loss carryforwards							$ 195,400,000
State and Local Jurisdiction | Research Tax Credit Carryforward
Components Of Income Tax Expense Benefit [Line Items]
Tax credit carryforward amount	$ 13,600,000	$ 14,300,000	$ 9,100,000