Leases - Summary of Cash and Non-cash Information Related to Operating Leases (Details) - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2025	Sep. 30, 2024
Leases [Abstract]
Cash paid for amounts included in measurement of lease liabilities	$ 13	$ 12

Cash paid for amounts included in measurement of lease liabilities.

No definition available.

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