Income Taxes - Additional Information (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Income Taxes [Line Items]
Increase (decrease) in valuation allowance	$ 39,900,000	$ 19,200,000
Federal operating loss carryforwards net	278,700,000
State operating loss carryforwards net	$ 225,700,000
Federal operating loss carryforwards expiry	2032
State operating loss carryforwards expiry	2028
Federal operating loss carryforwards expired net	$ 21,100,000
Federal research and development credits will begin to expire	2020
Unrecognized tax benefits	$ 25,259,000	9,714,000	$ 8,786,000
Unrecognized tax benefits that would affect the Company's effective tax rate	0	$ 0
Income tax penalties and interest expense, unrecognized tax benefits	0
Federal [Member]
Income Taxes [Line Items]
Research and development credit	11,500,000
State and Local [Member]
Income Taxes [Line Items]
Research and development credit	9,000,000
Orphan Drug [Member]
Income Taxes [Line Items]
Research and development credit	$ 46,900,000
California research and development credits expiration year	2034