Net Income (Loss) per Common Share- Antidilutive Securities Excluded From Computation of Diluted Net Loss per Common Share (Detail) - shares shares in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025	Jun. 30, 2024	Jun. 30, 2025	Jun. 30, 2024
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	330,177	832,233	378,017	832,233
Unvested RSUs
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	149,549	521,718	197,389	521,718
Employee Stock Option
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	178,764	310,515	178,764	310,515
ESPP Share Purchase Rights
Antidilutive Securities Excluded From Computation Of Earnings Per Share [Line Items]
Antidilutive securities excluded from computation of earnings per share	1,864	0	1,864	0

Atara Biotherapeutics, Inc. 10-Q