Segment Reporting - Schedule of Revenue and Long-lived Assets by Geographic Location (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Dec. 31, 2021	Jun. 30, 2021	Dec. 31, 2023	Dec. 31, 2022
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 1,554		$ 3,410	$ 3,132
Long-lived assets			439	406
Alight Holdings
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	1,554	$ 1,361
United States
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue			2,993	2,759
Long-lived assets			389	359
United States | Alight Holdings
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	1,358	1,168
Rest of World
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue			417	373
Long-lived assets			$ 50	$ 47
Rest of World | Alight Holdings
Revenues from External Customers and Long-Lived Assets [Line Items]
Revenue	$ 196	$ 193