Commitments and Contingencies - Additional Information (Details)	3 Months Ended
Mar. 31, 2020 lease renewal_option
Operating Leased Assets [Line Items]
Non-cancellable office leases \| lease	2
Operating lease, term of contract	1 year 6 months
Weighted-average discount rate	8.59%
One office lease
Operating Leased Assets [Line Items]
Number of one year renewal options \| renewal_option	3

Lessee, Operating Lease, Number

No definition available.

Name:	anab_LesseeOperatingLeaseNumber
Namespace Prefix:	anab_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Lessee, Operating Lease, One Year Renewal Options, Number

+ References

No definition available.

+ Details

Name:	anab_LesseeOperatingLeaseOneYearRenewalOptionsNumber
Namespace Prefix:	anab_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Term of lessee's operating lease, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeOperatingLeaseTermOfContract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average discount rate for operating lease calculated at point in time.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 4
-Subparagraph (g)(4)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918686-209980

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 55
-Paragraph 53
-URI http://asc.fasb.org/extlink&oid=77888399&loc=SL77918982-209971

+ Details

Name:	us-gaap_OperatingLeaseWeightedAverageDiscountRatePercent
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_OperatingLeasedAssetsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	anab_LeaseArrangementAxis=anab_OneOfficeLeaseMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: