Commitments and Contingencies - Additional Information (Detail) - USD ($)	Mar. 31, 2020	Dec. 31, 2019
Commitments And Contingencies Disclosure [Abstract]
Accrued termination charges	$ 0	$ 0
Liabilities related to indemnification agreements	$ 0	$ 0

Liabilities related to indemnification agreements.

No definition available.

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