Commitments and Contingencies - Additional Information (Detail) - USD ($)	Sep. 30, 2021	Dec. 31, 2020
Commitments And Contingencies Disclosure [Abstract]
Contractual obligations due to Bayer, MSK and QIMR	$ 0	$ 0
Accrued termination charges	600,000	0
Liabilities related to indemnification agreements	$ 0	$ 0

Carrying amount as of the balance sheet date of obligations due all licensors.

No definition available.

Name:	atra_DueToLicensorsCurrentAndNoncurrent
Namespace Prefix:	atra_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant