Segment Reporting - Schedule of Current Reportable Segments by Revenue (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended	9 Months Ended
	Sep. 30, 2022	Sep. 30, 2021	Jun. 30, 2021	Sep. 30, 2022
Segment Reporting Information [Line Items]
Revenue	$ 750	$ 690		$ 2,190
Alight Holdings
Segment Reporting Information [Line Items]
Revenue			$ 1,361
Employer Solutions
Segment Reporting Information [Line Items]
Revenue	645	587		1,882
Employer Solutions | Alight Holdings
Segment Reporting Information [Line Items]
Revenue			1,156
Employer Solutions | Recurring
Segment Reporting Information [Line Items]
Revenue	583	522		1,712
Employer Solutions | Recurring | Alight Holdings
Segment Reporting Information [Line Items]
Revenue			1,049
Employer Solutions | Project
Segment Reporting Information [Line Items]
Revenue	62	65		170
Employer Solutions | Project | Alight Holdings
Segment Reporting Information [Line Items]
Revenue			107
Professional Services
Segment Reporting Information [Line Items]
Revenue	95	93		276
Professional Services | Alight Holdings
Segment Reporting Information [Line Items]
Revenue			184
Professional Services | Recurring
Segment Reporting Information [Line Items]
Revenue	32	32		94
Professional Services | Recurring | Alight Holdings
Segment Reporting Information [Line Items]
Revenue			60
Professional Services | Project
Segment Reporting Information [Line Items]
Revenue	63	61		182
Professional Services | Project | Alight Holdings
Segment Reporting Information [Line Items]
Revenue			124
Hosted Business
Segment Reporting Information [Line Items]
Revenue	$ 10	$ 10		$ 32
Hosted Business | Alight Holdings
Segment Reporting Information [Line Items]
Revenue			$ 21