Income Taxes - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($)
Operating Loss Carryforwards [Line Items]
Increase (decrease) in valuation allowance	$ 41.0
Federal research and development credits expiration year start	2038
Decrease in deferred tax asset	$ 0.8
Unrecognized tax benefits that would not impact effective tax rate	2.6
Orphan Drug Credit
Operating Loss Carryforwards [Line Items]
Research and development credit	$ 9.6
Maximum
Operating Loss Carryforwards [Line Items]
Net operating losses carryforward available to offset future federal taxable income , percentage	80.00%
Federal
Operating Loss Carryforwards [Line Items]
Net operating losses (“NOL”) carryforwards	$ 175.8
Net operating loss carry forwards expiration year start	2034
Net operating losses (“NOL”) that can be carried forward indefinitely	$ 172.9
Research and development credit	9.6
California
Operating Loss Carryforwards [Line Items]
Net operating losses (“NOL”) carryforwards	$ 1.1
Net operating loss carry forwards expiration year start	2036
Research and development credit	$ 2.7
Irish
Operating Loss Carryforwards [Line Items]
Net operating losses (“NOL”) carryforwards	$ 53.0