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Income Taxes - Reconciliation Between Effective Tax Rate and Statutory Rate (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effective Income Tax Rate Reconciliation, Amount [Abstract]
Income taxes at statutory rates		$ (26.6)	$ (17.0)	$ (22.7)
State income tax, net of federal benefit		(5.5)	(0.7)	1.2
Permanent items		1.3	0.7	0.8
Research and development credits		(11.7)	(13.3)	(11.7)
Foreign rate differential		3.7	5.4	4.5
Stock and officers compensation		(5.1)	(10.4)	4.0
Rate change		0.0	(0.1)	(0.1)
Unrecognized tax benefits		0.0	(15.4)	27.7
Cumulative Effect of New Accounting Principle in Period of Adoption		(13.3)	0.0	0.0
Impact of Tax Cuts and Jobs Act of 2017		(0.4)	105.7	0.0
Other		1.3	(2.2)	0.0
Change in valuation allowance		56.9	(51.1)	(3.0)
Income taxes at effective rates	$ 1.6	$ 0.6	$ 1.6	$ 0.7

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