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Income Taxes - Reconciliation of Statutory Federal Income Tax Rate to Company's Effective Income Tax Rate (Detail)	12 Months Ended
	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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