Reconciliation of Income Tax Expense (Benefit) (Detail)	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax Disclosure [Abstract]
Federal income tax at statutory rates	21.00%	34.00%	34.00%
State income tax, net of federal tax benefit	4.00%	4.00%	1.00%
Research and development credits	2.00%	3.00%	3.00%
Stock compensation	(1.00%)	(1.00%)	(1.00%)
Channel rights	0.00%	0.00%	(25.00%)
Research and development true-up	0.00%	(7.00%)	0.00%
Officers compensation	(1.00%)	(2.00%)	0.00%
Other	(2.00%)	(3.00%)	0.00%
Federal rate change	3.00%	(124.00%)	0.00%
Change in valuation allowance	(26.00%)	96.00%	(12.00%)
Effective tax rate	0.00%	0.00%	0.00%

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

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