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Effective Income Tax Rate (Detail)
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Income Tax Disclosure [Abstract]      
Federal tax benefit rate 35.00% 35.00% 35.00%
Permanent items 6.20% (1.90%) 0.00%
Intercompany transfer of intellectual property 0.00% 0.90% (1.00%)
2017 tax law changes (265.00%) 0.00% 0.00%
Stock based compensation (5.00%) 0.00% 0.00%
Tax attribute expirations (2.80%) 0.00% 0.00%
Valuation allowance 231.60% (34.00%) (34.00%)
Effective income tax rate 0.00% 0.00% 0.00%