Income Taxes - Summary of Effective Tax Rates Differ from Statutory Income Tax Rates (Detail)	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Effective Income Tax Rate Reconciliation, Percent [Abstract]
Statutory income tax rate	(34.00%)	(34.00%)	(34.00%)
State income taxes	(6.10%)	(6.80%)	(5.70%)
Valuation allowance increase	(102.00%)	35.90%	35.20%
Effect of foreign operations			(1.30%)
Change in unused net operating loss and credit carryforwards	(4.00%)	3.70%	2.80%
Nondeductible items	1.60%	1.60%	3.90%
Impact of Tax Cuts and Jobs Act	144.50%
Effective income tax rate, Total	(0.00%)	0.40%	0.90%

Effective Income Tax Rate Reconciliation Expiration Of Unused Net Operating Loss And Credit Carry Forwards

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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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