Segment Reporting - Schedule of Current Reportable Segments by Revenue (Details) - USD ($) $ in Millions	6 Months Ended		12 Months Ended
	Dec. 31, 2021	Jun. 30, 2021	Dec. 31, 2023	Dec. 31, 2022
Segment Reporting Information [Line Items]
Revenue	$ 1,554		$ 3,410	$ 3,132
Alight Holdings
Segment Reporting Information [Line Items]
Revenue	1,554	$ 1,361
Other
Segment Reporting Information [Line Items]
Revenue			26	43
Other | Alight Holdings
Segment Reporting Information [Line Items]
Revenue	21	21
Operating Segments | Employer Solutions
Segment Reporting Information [Line Items]
Revenue			2,963	2,718
Operating Segments | Employer Solutions | Alight Holdings
Segment Reporting Information [Line Items]
Revenue	1,347	1,156
Operating Segments | Employer Solutions | Recurring
Segment Reporting Information [Line Items]
Revenue			2,695	2,467
Operating Segments | Employer Solutions | Recurring | Alight Holdings
Segment Reporting Information [Line Items]
Revenue	1,213	1,049
Operating Segments | Employer Solutions | Project
Segment Reporting Information [Line Items]
Revenue			268	251
Operating Segments | Employer Solutions | Project | Alight Holdings
Segment Reporting Information [Line Items]
Revenue	134	107
Operating Segments | Professional Services
Segment Reporting Information [Line Items]
Revenue			421	371
Operating Segments | Professional Services | Alight Holdings
Segment Reporting Information [Line Items]
Revenue	186	184
Operating Segments | Professional Services | Recurring
Segment Reporting Information [Line Items]
Revenue			142	128
Operating Segments | Professional Services | Recurring | Alight Holdings
Segment Reporting Information [Line Items]
Revenue	65	60
Operating Segments | Professional Services | Project
Segment Reporting Information [Line Items]
Revenue			279	243
Operating Segments | Professional Services | Project | Alight Holdings
Segment Reporting Information [Line Items]
Revenue	121	124
Operating Segments | Total Reportable Segments
Segment Reporting Information [Line Items]
Revenue			$ 3,384	$ 3,089
Operating Segments | Total Reportable Segments | Alight Holdings
Segment Reporting Information [Line Items]
Revenue	$ 1,533	$ 1,340