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Income Taxes - Income Tax Rate Reconciliation (Details) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2018
Dec. 31, 2017
Dec. 31, 2016
Income Tax Disclosure [Abstract]      
Tax benefit at federal statutory rate $ 100.8 $ 37.9 $ 69.7
State taxes, net of federal benefit 10.7 1.7 2.2
Foreign rate differential 1.8 (12.3) (38.8)
Research and other credits 86.5 25.4 12.6
Permanent differences (18.4) (9.0) (3.5)
Tax Cuts and Jobs Act impact 0.0 (61.7) 0.0
Change in valuation allowance (240.7) 38.9 (40.7)
Stock-based compensation 57.3 (20.1) (4.8)
Other nondeductible items (2.8) (1.0) (1.9)
Provision for income taxes $ (4.8) $ (0.2) $ (5.2)