Subsequent Event (Details) - Subsequent event ft² in Thousands	May 04, 2020 ft² $ / ft²
Subsequent Event [Line Items]
Area of leased property (sqft) \| ft²	45
Lease term	124 months
Lease renewal term	5 years
Lease termination term	7 years
Monthly base rate (usd per sqft) \| $ / ft²	4.20
Increase in annual rent	3.00%

Annual Rent Increase, Percent

No definition available.

Name:	anab_AnnualRentIncreasePercent
Namespace Prefix:	anab_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Lessee, Operating Lease, Lease Not yet Commenced, Expiration Period

+ References

No definition available.

+ Details

Name:	anab_LesseeOperatingLeaseLeaseNotyetCommencedExpirationPeriod
Namespace Prefix:	anab_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Rent Per Square Foot

+ References

No definition available.

+ Details

Name:	anab_RentPerSquareFoot
Namespace Prefix:	anab_
Data Type:	srt-types:perUnitItemType
Balance Type:	na
Period Type:	duration

- Definition

Area of a real estate property.

+ References

No definition available.

+ Details

Name:	us-gaap_AreaOfRealEstateProperty
Namespace Prefix:	us-gaap_
Data Type:	num:areaItemType
Balance Type:	na
Period Type:	instant

- Definition

Term of lessee's operating lease renewal for lease not yet commenced, 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 (b)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedRenewalTerm1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Term of lessee's operating lease not yet commenced, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents reported fact of one year, five months, and thirteen days.

+ References

+ Details

Name:	us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	instant

- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: