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Income Taxes - Deferred Tax Assets and Liabilities (Detail) - USD ($)
$ in Millions
Dec. 31, 2018
Dec. 31, 2017
Deferred tax assets:    
Net operating loss carryforwards $ 162.0 $ 188.7
Capitalized research and development expenses 62.1 8.4
Tax credits 59.0 47.8
Share-based compensation 12.5 13.8
Fixed and intangible assets 16.0 0.4
Accrued liabilities and reserves 22.5 20.5
Total gross deferred tax assets 334.1 279.6
Less: valuation allowance (330.1) (263.5)
Total net deferred tax assets 4.0 16.1
Deferred tax liabilities:    
Fixed assets and acquired intangibles assets (3.8) (0.1)
Convertible debt discount (0.1) (15.9)
Total deferred tax liabilities (3.9) (16.0)
Net deferred tax assets (liabilities) $ 0.1 $ 0.1