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Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2020
Dec. 31, 2019
Deferred tax assets:    
Net operating loss carryforwards $ 69.2 $ 127.4
Capitalized research and development expenses 53.6 57.1
Tax credits 101.1 78.6
Share-based compensation 13.0 10.9
Fixed and intangible assets 16.9 14.0
Accrued liabilities and reserves 110.3 62.0
Convertible debt 0.0 1.7
Total gross deferred tax assets 364.1 351.7
Less: valuation allowance (55.5) (332.2)
Total net deferred tax assets 308.6 19.5
Deferred tax liabilities:    
Fixed assets and acquired intangibles assets (36.3) (19.6)
Convertible debt discount (55.9) 0.0
Total deferred tax liabilities (92.2) (19.6)
Net deferred tax assets (liabilities) $ 216.4  
Net deferred tax assets (liabilities)   $ (0.1)