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Stockholders' Equity (Detail) - shares
Mar. 31, 2019
Dec. 31, 2018
Common Stock, Capital Shares Reserved for Future Issuance 33,371,464 33,432,421
Warrant [Member]    
Common Stock, Capital Shares Reserved for Future Issuance 17,678,074 17,732,700
2003 Stock Incentive Plan    
Common Stock, Capital Shares Reserved for Future Issuance 6,584,397 6,590,725
2018 Equity Incentive Plan    
Common Stock, Capital Shares Reserved for Future Issuance 9,108,996 9,108,996