Earnings Per Share - Summary of Basic and Diluted (Net Loss) Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2023	Jun. 30, 2022	Jun. 30, 2023	Jun. 30, 2022
Numerator
Net (loss) income attributable to Alight, Inc. - basic	$ (67)	$ 51	$ (135)	$ 40
Loss impact of conversion of noncontrolling interest		1		(1)
Net (loss) income attributable to Alight, Inc. - diluted	$ (67)	$ 52	$ (135)	$ 39
Denominator
Weighted-average shares outstanding - basic	490,306,205	457,851,348	483,358,533	457,347,581
Dilutive effect of the exchange of noncontrolling interest units		75,886,716		75,886,716
Dilutive effect of RSUs				836,356
Weighted-average shares outstanding - diluted	490,306,205	533,738,064	483,358,533	534,070,653
Basic (net loss) earnings per share	$ (0.14)	$ 0.11	$ (0.28)	$ 0.09
Diluted (net loss) earnings per share	$ (0.14)	$ 0.10	$ (0.28)	$ 0.07