Leases - Summary of Cash and Non-cash Information Related to Operating Leases (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Leases [Abstract]
Cash paid for amounts included in measurement of lease liabilities	$ 17	$ 17	$ 15
Cash received from tenant improvement allowances	0	0	9
Recognition of tenant improvement allowance receivable included in Other current liabilities	$ 0	$ 0	$ 4

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