Earnings Per Share - Summary of Basic and Diluted (Net Loss) Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	6 Months Ended	12 Months Ended
	Dec. 31, 2021	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Numerator
Net (loss) income attributable to Alight, Inc. - basic	$ (35)	$ (345)	$ (62)	$ (35)
Denominator
Weighted-average shares outstanding - basic		489,461,259	458,558,192	439,800,624
Basic (net loss) earnings per share	$ (0.08)	$ (0.7)	$ (0.14)	$ (0.08)
Diluted (net loss) earnings per share	$ (0.08)	$ (0.7)	$ (0.14)	$ (0.08)