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Income Tax - Additional Information (Detail) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017 | Dec. 31, 2016 | Dec. 31, 2015
Income Taxes [Line Items] |  |  |  |
Future enacted income tax rate assumed for both Federal and state deferred tax calculations |  | 34.00% | 34.00% | 34.00%
Net operating loss carryforwards |  | $ 9,900 | $ 1,300 |
Unclaimed tax deductions |  | $ 15,500 |  |
Starting range of research and development tax credits expiration period |  | 2018 |  |
Ending range of research and development tax credits expiration period |  | 2037 |  |
Deferred tax assets recognized |  | $ 0 |  |
Unrecognized tax benefits |  | 700 |  |
Accrued interest and penalties related to income taxes |  | $ 0 | $ 0 |
Income tax rate assumed for both State and Foreign deferred tax calculations |  | 34.00% | 34.00% | 34.00%
Scenario, Forecast |  |  |  |
Income Taxes [Line Items] |  |  |  |
Income tax rate assumed for both State and Foreign deferred tax calculations | 21.00% |  |  |
Investment tax credits carryforwards [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Investment tax credits |  | $ 2,603 |  |
Research [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Investment tax credits |  | 19,000,000 |  |
CANADA | Investment tax credits carryforwards [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Investment tax credits |  | 2,600 | $ 2,600 |
CANADA | Non Capital Loss Carryforwards [Member] |  |  |  |
Income Taxes [Line Items] |  |  |  |
Investment tax credits |  | 120,400 | 115,900 |
UNITED KINGDOM |  |  |  |
Income Taxes [Line Items] |  |  |  |
Net operating loss carryforwards |  | $ 849 | $ 200 |

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- Definition Amount, after allocation of valuation allowances and deferred tax liability, of deferred tax asset attributable to deductible differences and carryforwards, without jurisdictional netting. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319 + Details
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