Summary of Significant Accounting Policies (Potential Common Stock Equivalents Excluded from Calculation of Diluted Net Loss per Share) (Detail) - shares	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common stock equivalents excluded from the calculation of diluted net loss per share	187,165,250	110,390,606
Option [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common stock equivalents excluded from the calculation of diluted net loss per share	37,432,023	22,089,527
Warrant [Member]
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Potential common stock equivalents excluded from the calculation of diluted net loss per share	149,733,227	88,301,079