Restatement of Previously Issued Financial Statements - Comprehensive Loss (Details) - USD ($) $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2024	Jun. 30, 2024	Mar. 31, 2024	Sep. 30, 2023	Jun. 30, 2023	Mar. 31, 2023	Sep. 30, 2024	Sep. 30, 2023
Reclassification [Line Items]
Net loss	$ (47,442)			$ (24,269)			$ (190,340)	$ (106,986)
Net unrealized gain on marketable debt securities	2,452			1,151			510	6,244
Comprehensive income (loss)	(44,990)	$ (69,280)	$ (75,561)	(23,118)	$ (20,189)	$ (57,436)	(189,830)	(100,742)
Comprehensive loss attributable to non-controlling interest	(1,154)			0			(3,275)	0
Comprehensive loss attributable to Xencor, Inc.	(43,836)			$ (23,118)			(186,555)	$ (100,742)
As Reported
Reclassification [Line Items]
Net loss	(46,297)						(182,414)
Net unrealized gain on marketable debt securities	2,452						510
Comprehensive income (loss)	(43,845)						(181,904)
Comprehensive loss attributable to non-controlling interest	(1,154)						(3,275)
Comprehensive loss attributable to Xencor, Inc.	(42,691)						(178,629)
Adjustment
Reclassification [Line Items]
Net loss	(1,145)						(7,926)
Net unrealized gain on marketable debt securities	0						0
Comprehensive income (loss)	(1,145)						(7,926)
Comprehensive loss attributable to non-controlling interest	0						0
Comprehensive loss attributable to Xencor, Inc.	$ (1,145)						$ (7,926)