Earnings Per Share - Summary of Basic and Diluted (Net Loss) Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Numerator
Net Income (Loss) From Continuing Operations	$ (1,055)	$ (44)	$ (2,145)	$ (169)
Less: Net income (loss) attributable to noncontrolling interest	1	0	2	2
Net Income (loss) from continuing operations attributable to Alight, Inc.	(1,054)	(44)	(2,143)	(167)
Net Income (Loss) From Discontinued Operations, Net of Tax	(13)	(30)	(22)	2
Net Income (Loss) Attributable to Alight, Inc.	(1,067)	(74)	(2,165)	(165)
Loss impact of conversion of noncontrolling interest	0	0	0	(1)
Net income (loss) attributable to Alight, Inc. - diluted	$ (1,067)	$ (74)	$ (2,165)	$ (166)
Denominator
Weighted-average shares outstanding - basic (in shares)	526,576,757	535,828,896	529,206,657	545,659,335
Dilutive effect of the exchange of noncontrolling interest units (in shares)	0	0	0	560,433
Dilutive effect of RSUs (in shares)	0	0	0	0
Weighted-average shares outstanding - diluted (in shares)	526,576,757	535,828,896	529,206,657	546,219,768
Basic and Diluted (net loss) earnings per share
Continuing operations, basic (in dollars per share)	$ (2.00)	$ (0.08)	$ (4.05)	$ (0.31)
Continuing operations, diluted (in dollars per share)	(2.00)	(0.08)	(4.05)	(0.31)
Discontinued operations, basic (in dollars per share)	(0.02)	(0.06)	(0.04)	0.00
Discontinued operations, diluted (in dollars per share)	(0.02)	(0.06)	(0.04)	0.00
Net income (loss), basic (in dollars per share)	(2.02)	(0.14)	(4.09)	(0.31)
Net income (loss), diluted (in dollars per share)	$ (2.02)	$ (0.14)	$ (4.09)	$ (0.31)