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, 2022	Sep. 30, 2021	Sep. 30, 2022	Sep. 30, 2021
Numerator
Net (loss) income attributable to Alight, Inc. - basic	$ (37)	$ (107)	$ 3	$ (107)
Loss impact of conversion of noncontrolling interest			(1)
Net (loss) income attributable to Alight, Inc. - diluted	$ (37)	$ (107)	$ 2
Denominator
Weighted-average shares outstanding - basic	457,904,703	438,968,920	457,535,329
Dilutive effect of the exchange of noncontrolling interest units			75,800,317
Dilutive effect of RSUs			770,953
Weighted-average shares outstanding - diluted	457,904,703	438,968,920	534,106,599
Basic (net loss) earnings per share	$ (0.08)	$ (0.24)	$ 0.01
Diluted (net loss) earnings per share	$ (0.08)	$ (0.24)	$ 0.00