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Income Taxes - Additional Information (Detail) - USD ($) | 12 Months Ended |  |
--- | --- | --- | ---
Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015 | Dec. 31, 2007
Income Tax [Line Items] |  |  |  |  |
Gross federal R&D tax credit-related deferred tax assets derecognized |  | $ 6,100,000 |  |  |
Unrecognized tax benefit |  | 1,475,000 | $ 1,539,000 | $ 1,314,000 |
Tax-related penalties or interest recognized |  | $ 0 | $ 0 |  |
Corporate tax rate |  | 34.00% | 34.00% |  |
Decrease in deferred income tax assets |  | $ 7,300,000 |  |  |
Decrease in valuation allowance |  | 7,300,000 |  |  |
Net effect of tax reform enactment |  | 0 |  |  |
Scenario Forecast |  |  |  |  |
Income Tax [Line Items] |  |  |  |  |
Corporate tax rate | 21.00% |  |  |  |
Federal Income Tax |  |  |  |  |
Income Tax [Line Items] |  |  |  |  |
Net operating loss carryforwards |  | 54,800,000 | $ 68,300,000 |  | $ 108,900,000
Tax credit carryforwards available to offset future federal tax liabilities |  | $ 100,000 |  |  |
Credit carryforward, expire period |  | Dec. 31, 2037 |  |  |
Net operating loss carryforward,derecognized |  |  |  |  | $ 22,900,000
Federal Income Tax | Beginning Year |  |  |  |  |
Income Tax [Line Items] |  |  |  |  |
Net operating loss carryforwards expiration year |  | 2025 |  |  |
Federal Income Tax | Ending Year |  |  |  |  |
Income Tax [Line Items] |  |  |  |  |
Net operating loss carryforwards expiration year |  | 2038 |  |  |
State Income Tax |  |  |  |  |
Income Tax [Line Items] |  |  |  |  |
Net operating loss carryforwards |  | $ 41,100,000 | $ 46,600,000 |  |
Tax credit carryforwards available to offset future federal tax liabilities |  | $ 4,400,000 |  |  |
State Income Tax | Beginning Year |  |  |  |  |
Income Tax [Line Items] |  |  |  |  |
Net operating loss carryforwards expiration year |  | 2028 |  |  |
State Income Tax | Ending Year |  |  |  |  |
Income Tax [Line Items] |  |  |  |  |
Net operating loss carryforwards expiration year |  | 2038 |  |  |
CALIFORNIA |  |  |  |  |
Income Tax [Line Items] |  |  |  |  |
Net operating loss carryforwards |  | $ 37,700,000 |  |  |
Net operating loss carryforward,derecognized |  | $ 0 |  |  |

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- Definition Percentage of domestic federal statutory tax rate applicable to pretax income (loss). + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 12 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32687-109319 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SAB TOPIC 6.I) -URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 13 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32698-109319 + Details
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