Income Taxes - Additional Information (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Line Items]
Increase (decrease) in deferred tax valuation allowance	$ 19.7	$ (12.6)	$ 40.7
Income tax, federal statutory rate	21.00%	35.00%	35.00%
Decrease related to net federal deferred tax assets	$ 35.6
Offsetting change in valuation allowance	35.6
Accrued interest and penalties related to unrecognized tax benefits	0.0	$ 0.0	$ 0.0
Federal Tax Authority [Member]
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 193.3
Operating loss carryforward expiration year	2030
Tax credit carryforwards	$ 47.0
Federal Tax Authority [Member] | Research and Development Tax Credit [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 8.7
Tax credit carryforward expiration year	2027
State Tax Authority [Member]
Income Tax Disclosure [Line Items]
Operating loss carryforward expiration year	2025
Other net operating losses	$ 2.5
State Tax Authority [Member] | California
Income Tax Disclosure [Line Items]
Net operating loss carryforwards	$ 88.3
Operating loss carryforward expiration year	2030
State Tax Authority [Member] | Research and Development Tax Credit [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards	$ 6.9
State Tax Authority [Member] | Minimum Tax Credit Carryforward [Member]
Income Tax Disclosure [Line Items]
Tax credit carryforwards			$ 0.1
Foreign Tax Authority [Member]
Income Tax Disclosure [Line Items]
Foreign tax credit carryforwards	$ 1.2
Operating loss carryforward expiration year	2027