Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax [Line Items]
Total provision (benefit) for income taxes	$ 0	$ 0	$ 0
Qualified clinical testing expenses for orphan drug credit	25.00%
Increase (Decrease) in valuation allowance	$ 82,100,000	(2,200,000)
Unrecognized income tax benefits that, if recognized, would affect effective tax rate	0	0
Unrecognized income tax benefits	25,421,000	20,730,000	$ 13,865,000	$ 3,228,000
Accrued interest or penalty related to uncertain tax benefits	$ 0	$ 0
U.S. federal statutory tax rate	21.00%	34.00%	34.00%
Federal
Income Tax [Line Items]
Net operating loss carryforwards	$ 1,040,800,000
Federal | Minimum
Income Tax [Line Items]
Net operating loss carryforwards, expiration year	2024
Federal | Maximum
Income Tax [Line Items]
Net operating loss carryforwards, expiration year	2038
Federal | Research Tax Credit Carryforward
Income Tax [Line Items]
Tax credits carryforwards	$ 19,400,000
Federal | Orphan Drug Credit
Income Tax [Line Items]
Tax credits carryforwards	141,500,000
California
Income Tax [Line Items]
Net operating loss carryforwards	$ 216,000,000
California | Minimum
Income Tax [Line Items]
Net operating loss carryforwards, expiration year	2018
California | Maximum
Income Tax [Line Items]
Net operating loss carryforwards, expiration year	2033
California | Research Tax Credit Carryforward
Income Tax [Line Items]
Tax credits carryforwards	$ 10,600,000