Additional clinical and milestone payments received under license agreement.

No definition available.

Name:	rcus_AdditionalClinicalAndMilestonePaymentsReceivedUnderLicenseAgreement
Namespace Prefix:	rcus_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Additional clinical regulatory and commercialization milestone payments.

+ References

No definition available.

+ Details

Name:	rcus_AdditionalClinicalRegulatoryAndCommercializationMilestonePayments
Namespace Prefix:	rcus_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Clinical regulatory and commercialization milestone payments.

+ References

No definition available.

+ Details

Name:	rcus_ClinicalRegulatoryAndCommercializationMilestonePayments
Namespace Prefix:	rcus_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Clinical Regulatory Milestone Payments

+ References

No definition available.

+ Details

Name:	rcus_ClinicalRegulatoryMilestonePayments
Namespace Prefix:	rcus_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Contingent milestone payments receivable.

+ References

No definition available.

+ Details

Name:	rcus_ContingentMilestonePaymentsReceivable
Namespace Prefix:	rcus_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

License and collaboration agreements.

+ References

No definition available.

+ Details

Name:	rcus_LicenseAndCollaborationAgreementsLineItems
Namespace Prefix:	rcus_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Milestone expense.

+ References

No definition available.

+ Details

Name:	rcus_MilestoneExpense
Namespace Prefix:	rcus_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Milestone payments.

+ References

No definition available.

+ Details

Name:	rcus_MilestonePayments
Namespace Prefix:	rcus_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Non refundable and non creditable cash payments.

+ References

No definition available.

+ Details

Name:	rcus_NonRefundableAndNonCreditableCashPayments
Namespace Prefix:	rcus_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Option period.

+ References

No definition available.

+ Details

Name:	rcus_OptionPeriod
Namespace Prefix:	rcus_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Payment for option exercise.

+ References

No definition available.

+ Details

Name:	rcus_PaymentForOptionsExercised
Namespace Prefix:	rcus_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Reimbursement of research and development expense

+ References

No definition available.

+ Details

Name:	rcus_ReimbursementOfResearchAndDevelopmentExpense
Namespace Prefix:	rcus_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Remaining Milestone Payments

+ References

No definition available.

+ Details

Name:	rcus_RemainingMilestonePayments
Namespace Prefix:	rcus_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Royalties Payable Term

+ References

No definition available.

+ Details

Name:	rcus_RoyaltiesPayableTerm
Namespace Prefix:	rcus_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time, classified as current.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-11

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479837/606-10-45-3

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479837/606-10-45-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-8

+ Details

Name:	us-gaap_ContractWithCustomerAssetNetCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of liabilities classified as other.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477657/942-210-S45-1

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478478/942-210-S50-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(15)(4))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(15)(3))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(15))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(24))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480657/210-10-S50-8

Reference 8: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(23))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a)(5))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 10: http://www.xbrl.org/2003/role/exampleRef
-Topic 480
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 64
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481620/480-10-55-64

Reference 11: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 605
-Name Accounting Standards Codification
-Section 55
-Paragraph 14
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477548/944-605-55-14

Reference 12: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 605
-Name Accounting Standards Codification
-Section 55
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477548/944-605-55-11

Reference 13: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(15))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

Reference 14: http://www.xbrl.org/2003/role/exampleRef
-Topic 946
-SubTopic 830
-Name Accounting Standards Codification
-Section 55
-Paragraph 12
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479168/946-830-55-12

Reference 15: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(12)(b)(2))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479170/946-210-S99-1

Reference 16: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(12)(b)(3))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479170/946-210-S99-1

Reference 17: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-04(12)(b)(1))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479170/946-210-S99-1

Reference 18: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 944
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03(a)(17))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478777/944-210-S99-1

+ Details

Name:	us-gaap_OtherLiabilities
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of liabilities classified as other, due within one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S50
-Paragraph 8
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480657/210-10-S50-8

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(19)(a)(5))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(20))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Topic 852
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 10
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481372/852-10-55-10

+ Details

Name:	us-gaap_OtherLiabilitiesCurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of liabilities classified as other, due after one year or the normal operating cycle, if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(24))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 210
-SubTopic 10
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02(23))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480566/210-10-S99-1

+ Details

Name:	us-gaap_OtherLiabilitiesNoncurrent
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of expense for research and development. Excludes cost for computer software product to be sold, leased, or otherwise marketed, writeoff of research and development assets acquired in transaction other than business combination or joint venture formation or both, and write-down of intangible asset acquired in business combination or from joint venture formation or both, used in research and development activity.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 730
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482916/730-10-50-1

+ Details

Name:	us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_TypeOfArrangementAxis=rcus_TaihoCollaborationAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=rcus_AstraZenecaAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=rcus_TaihoPharmaceuticalCoLtdMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=rcus_OptionAndLicenseAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementScenarioAxis=srt_ScenarioForecastMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=rcus_STAR221DevelopmentActivitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=rcus_STAR121DevelopmentActivitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=rcus_PRISM1DevelopmentActivitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=rcus_WuXiBiologicsLicenseAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=rcus_AntiPDOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=rcus_AntiCDThirtyNineMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_CounterpartyNameAxis=rcus_AbmunoLicenseAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

License and Collaborations (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended		12 Months Ended
	Mar. 31, 2027	Sep. 30, 2026	Jun. 30, 2026	Dec. 31, 2025	Sep. 30, 2025	Mar. 31, 2025	Sep. 30, 2026	Sep. 30, 2025	Dec. 31, 2024	Dec. 31, 2023	Dec. 31, 2020	Dec. 31, 2017
License And Collaboration Agreements [Line Items]
Receivable from collaboration partners		$ 18		$ 11			$ 18
Other current liabilities		141		154			141
Other noncurrent liabilities		125		135			125
Taiho Collaboration Agreement
License And Collaboration Agreements [Line Items]
Milestone payments			$ 30
Receivable from collaboration partners		18		10			18
Other liabilities		32		44			32
AstraZeneca Agreement
License And Collaboration Agreements [Line Items]
Milestone payments											$ 18
Development cost recorded within research and development expenses		12			$ 7		21	$ 16
Other current liabilities		33					33
Other noncurrent liabilities				12
Taiho Pharmaceutical Co., Ltd \| Taiho Agreement
License And Collaboration Agreements [Line Items]
Option period												5 years
Non refundable and non creditable cash payments												$ 35
Payment for option exercise				15
Royalties payable term												10 years
Reimbursement of research and development expense		5			7		16	24
Taiho Pharmaceutical Co., Ltd \| Taiho Agreement \| Minimum
License And Collaboration Agreements [Line Items]
Payment for option exercise												$ 3
Taiho Pharmaceutical Co., Ltd \| Taiho Agreement \| Maximum
License And Collaboration Agreements [Line Items]
Payment for option exercise												15
Additional clinical and regulatory milestone payments receivable		15		172								130
Contingent milestone payments receivable				$ 145								$ 145
Taiho Pharmaceutical Co., Ltd \| Taiho Agreement \| Maximum \| Forecast
License And Collaboration Agreements [Line Items]
Remaining milestone payments	$ 15
STAR-221 Development Activities \| Taiho Collaboration Agreement
License And Collaboration Agreements [Line Items]
Milestone payments										$ 28
STAR-121 Development Activities \| Taiho Collaboration Agreement
License And Collaboration Agreements [Line Items]
Milestone payments									$ 26
PRISM-1 Development Activities \| Taiho Collaboration Agreement
License And Collaboration Agreements [Line Items]
Milestone payments						$ 19
WuXi Biologics License Agreement \| anti-PD-1
License And Collaboration Agreements [Line Items]
Development milestone expense		0			0		0	0
WuXi Biologics License Agreement \| anti-CD39
License And Collaboration Agreements [Line Items]
Development milestone expense		0			0		0	0
WuXi Biologics License Agreement \| Maximum \| anti-PD-1
License And Collaboration Agreements [Line Items]
Clinical regulatory milestone payments							50
Clinical, regulatory and commercialization milestone payments							375
WuXi Biologics License Agreement \| Maximum \| anti-CD39
License And Collaboration Agreements [Line Items]
Additional clinical, regulatory and commercialization milestone payments							14
Abmuno License Agreement
License And Collaboration Agreements [Line Items]
Development milestone expense		$ 0			$ 0		0	$ 0
Additional clinical, regulatory and commercialization milestone payments							$ 88