Income Taxes - Additional Information (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Taxes [Line Items]
Effective income tax rate and tax provision from continuing operations	0.00%	0.00%	0.00%
Federal statutory income tax rate	21.00%	34.00%	34.00%
Tax Cuts and Jobs Act, income tax expense (benefit)	$ 0
Deferred tax assets, relates to net operating loss carryforwards	49,689,000	$ 22,246,000
Deferred tax assets, operating loss foreign	177,000,000
Deferred tax assets, operating loss state	$ 179,400,000
Operating loss expiration date	2032
Research and development tax credit	$ 12,908,000	8,631,000
Increase in valuation allowance	$ 25,700,000	13,700,000	$ 4,900,000
Time period over percentage change of ownership	3 years
Unrecognized tax benefits related to liability	$ 0	0
Unrecognized tax benefits related to penalties or interest accrued	0	0
Unrecognized tax benefits that would impact effective tax rate	0
Reserve for unrecognized tax benefits	$ 3,810,000	$ 2,471,000	$ 1,474,000	$ 719,000
Internal Revenue Service
Income Taxes [Line Items]
Operating loss carryforwards adjustments period	3 years
California Franchise Tax Board
Income Taxes [Line Items]
Operating loss carryforwards adjustments period	4 years
Australia
Income Taxes [Line Items]
Uncertain tax positions statute of limitation	4 years
Minimum
Income Taxes [Line Items]
Change of ownership percentage in equity ownership	50.00%
Federal
Income Taxes [Line Items]
Research and development tax credit	$ 4,500,000
Research and development tax credit carryforward, expiration year	2032
Orphan tax credit carryovers, expiration beginning year	2036
Orphan tax credit carryovers	$ 9,000,000
State
Income Taxes [Line Items]
Research and development tax credit	$ 3,800,000