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Income Taxes - Reconciliation of Statutory Federal Income Tax Rate to Company's Effective Income Tax Rate (Detail) | 12 Months Ended
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Dec. 31, 2018 | Dec. 31, 2017
Income Tax Disclosure [Line Items] |  |
U.S. federal statutory rate | 21.00% | 34.00%
State income taxes, net of federal benefit, Tax Rate | 6.00% | 5.00%
Other permanent differences, Tax Rate | (1.00%) | (1.00%)
Tax credits, Tax Rate | 3.00% | 5.00%
Other items, Tax Rate | (1.00%) |
Change in rate due to Tax Reform, Tax Rate |  | (27.00%)
Net change in valuation allowance, Tax Rate | (28.00%) | (33.00%)
Mirna therapeutics |  |
Income Tax Disclosure [Line Items] |  |
Mirna acquisition, Tax Rate |  | 17.00%

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- Definition 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. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -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 2: http://fasb.org/us-gaap/role/ref/legacyRef -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 3: http://fasb.org/us-gaap/role/ref/legacyRef -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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