Income Taxes - Income Tax Expense by Applying Statutory Federal Income Tax Rate (Detail)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Tax at statutory federal rate	21.00%	34.00%	34.00%
State tax, net of federal benefit	(0.40%)	(0.40%)	(0.40%)
Other	(4.90%)	(4.40%)	(3.70%)
Foreign rate differential	(1.00%)	0.50%	(0.20%)
Tax credits	2.80%	4.50%	3.20%
Excess tax benefits related to stock-based compensation	2.00%	21.50%
Effect of Tax Cuts and Jobs Act of 2017		(121.10%)
Change in valuation allowance	(21.20%)	61.40%	(38.40%)
Total	(1.70%)	(4.00%)	(5.50%)

Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to changes in the valuation allowance for deferred tax assets.

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to statutory income tax expense (benefit) outside of the country of domicile.

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to tax credits. Including, but not limited to, research credit, foreign tax credit, investment tax credit, and other tax credits.

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