Summary of Significant Accounting Policies - Income Taxes (Details) $ in Millions	Dec. 31, 2020 USD ($)
Domestic Tax Authority
Income Taxes [Line Items]
Income tax refund	$ 0.8

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.

No definition available.

Name:	xncr_IncomeTaxesLineItems
Namespace Prefix:	xncr_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration