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Income Taxes - Components of Deferred Tax Assets and Deferred Tax Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Deferred tax assets:    
Net operating loss carryforwards $ 264.8 $ 119.6
Research credit carryforwards 157.3 67.9
Stock-based compensation 11.1 20.0
Accruals and reserves 42.0 34.0
Fixed assets and intangible assets 0.7 0.0
Other 1.1 0.0
Gross deferred tax assets 477.0 241.5
Valuation allowance (476.0) (233.7)
Total deferred tax assets, net of valuation allowance 1.0 7.8
Deferred tax liabilities:    
Fixed assets and intangible assets 0.0 6.6
Other 0.0 0.4
Total deferred tax liability 0.0 7.0
Net deferred tax assets $ 1.0 $ 0.8