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Income Taxes - Deferred Tax Assets and Liabilities (Details) - USD ($)
$ in Millions
Dec. 31, 2019
Dec. 31, 2018
Deferred tax assets:    
Net operating loss carryforwards $ 127.4 $ 162.0
Capitalized research and development expenses 57.1 62.1
Tax credits 78.6 59.0
Share-based compensation 10.9 12.5
Fixed and intangible assets 14.0 16.0
Accrued liabilities and reserves 62.0 22.5
Convertible Debt 1.7 0.0
Total gross deferred tax assets 351.7 334.1
Less: valuation allowance (332.2) (330.1)
Total net deferred tax assets 19.5 4.0
Deferred tax liabilities:    
Fixed assets and acquired intangibles assets (19.6) (3.8)
Convertible debt discount 0.0 (0.1)
Total deferred tax liabilities (19.6) (3.9)
Net deferred tax assets (liabilities) $ (0.1) $ 0.1