Net Loss Per Share - Potentially Dilutive Securities Excluded from Calculation of Diluted Net Loss Attributable to Stockholders of Arena Per Share (Detail) - shares shares in Thousands	3 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss attributable to stockholders of Arena per share (shares)	31,731	19,405
Stock options
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss attributable to stockholders of Arena per share (shares)	31,643	19,091
RSUs and unvested restricted stock
Antidilutive Securities Excluded from Computation of Earnings Per Share [Line Items]
Total antidilutive securities excluded from calculation of diluted net loss attributable to stockholders of Arena per share (shares)	88	314