Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax [Line Items]
Total provision (benefit) for income taxes	$ 0	$ 0	$ 0
Increase (decrease) in valuation allowance	62,500,000	(82,100,000)
Unrecognized income tax benefits that, if recognized, would affect effective tax rate	0	0
Unrecognized income tax benefits	24,301,000	25,421,000	$ 20,730,000	$ 13,865,000
Accrued interest or penalty related to uncertain tax benefits	0	$ 0
Federal
Income Tax [Line Items]
Net operating loss carryforwards	1,225,500,000
Net operating loss carryforwards, subject to expiration	868,100,000
Net operating loss carryforwards, not subject to expiration	$ 357,400,000
Federal | Minimum
Income Tax [Line Items]
Net operating loss carryforwards, expiration year	2027
Federal | Maximum
Income Tax [Line Items]
Net operating loss carryforwards, expiration year	2037
Federal | Research Tax Credit Carryforward
Income Tax [Line Items]
Tax credits carryforwards	$ 19,300,000
Federal | Orphan Drug Credit
Income Tax [Line Items]
Tax credits carryforwards	$ 132,700,000
Federal | Research Tax Credit Carryforward and Orphan Drug Credit | Minimum
Income Tax [Line Items]
Net operating loss carryforwards, expiration year	2024
Federal | Research Tax Credit Carryforward and Orphan Drug Credit | Maximum
Income Tax [Line Items]
Net operating loss carryforwards, expiration year	2039
California
Income Tax [Line Items]
Net operating loss carryforwards	$ 208,000,000.0
California | Minimum
Income Tax [Line Items]
Net operating loss carryforwards, expiration year	2020
California | Maximum
Income Tax [Line Items]
Net operating loss carryforwards, expiration year	2039
California | Research Tax Credit Carryforward
Income Tax [Line Items]
Tax credits carryforwards	$ 12,000,000.0