Income Taxes - Income Tax Expense (Benefit) Differed from Amounts Computed by Applying Statutory Federal Income Tax Rate (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Income Tax Disclosure [Abstract]
Income tax provision at statutory rate	$ (17,013)	$ (9,955)	$ (6,021)
Nondeductible items	456	(32)	349
Change in tax credits	(678)	(893)	39
Change in valuation allowance	17,911	10,965	5,894
Other	(676)	(85)	(261)
Income tax expense (benefit)	$ 0us-gaap_IncomeTaxExpenseBenefit	$ 0us-gaap_IncomeTaxExpenseBenefit	$ 0us-gaap_IncomeTaxExpenseBenefit

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

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The amount of income tax expense or benefit for the period computed by applying the domestic federal statutory tax rates to pretax income from continuing operations.

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Amount 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 exempt income, equity in earnings (loss) of an unconsolidated subsidiary, minority noncontrolling interest income (loss), tax holiday, disposition of a business, disposition of an asset, repatriation of foreign earnings, repatriation of foreign earnings jobs creation act of 2004, increase (decrease) in enacted tax rate, prior year income taxes, increase (decrease) in deferred tax asset valuation allowance, and other adjustments.

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