Earnings Per Share - Summary of Basic and Diluted Earnings Per Share (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Jun. 30, 2021	Sep. 30, 2020
Numerator
Net loss attributable to Alight, Inc. - basic and diluted	$ (107)	$ (39)	$ (25)	$ (85)
Denominator
Weighted average shares outstanding - basic and diluted	438,968,920
Basic and diluted net loss per share	$ (0.24)