Debt - Additional Information (Details) - USD ($)	1 Months Ended						6 Months Ended		9 Months Ended	12 Months Ended
	Mar. 31, 2023	Aug. 31, 2021	Jul. 31, 2021	Aug. 31, 2020	May 31, 2020	May 31, 2017	Dec. 31, 2021	Jun. 30, 2021	Sep. 30, 2023	Dec. 31, 2023	Dec. 31, 2022
Debt Instrument [Line Items]
Debt balance										$ 2,794,000,000	$ 2,823,000,000
Amortization of financing fees and benefits							$ (2,000,000)			(2,000,000)	(2,000,000)
Interest expense related to debt instruments								$ 53,000,000		219,000,000	138,000,000
Alight Holdings
Debt Instrument [Line Items]
Amortization of financing fees and benefits								9,000,000
Interest expense related to debt instruments								105,000,000
Interest Expense
Debt Instrument [Line Items]
Amortization of financing fees and benefits							2,000,000	8,000,000		$ 2,000,000	$ 3,000,000
Revolving Credit Facility
Debt Instrument [Line Items]
Increase in revolving credit facility	$ 300,000,000
Revolving Credit Facility | Alight Holdings
Debt Instrument [Line Items]
Debt term						5 years
Maturity Date						May 01, 2022
Line of credit, maximum borrowing capacity						$ 250,000,000
$300m Revolving Credit Facility, Amended
Debt Instrument [Line Items]
Maturity Date		Aug. 31, 2026								Aug. 31, 2026
Line of credit, maximum borrowing capacity		$ 294,000,000								$ 300,000,000
Unused letters of credit										3,000,000
Proceeds from Lines of Credit										$ 0
$300m Revolving Credit Facility, Amended | Alight Holdings
Debt Instrument [Line Items]
Maturity Date				Oct. 31, 2024
Line of credit, maximum borrowing capacity				$ 226,000,000
Term Loan
Debt Instrument [Line Items]
Maturity Date										May 01, 2024
Repayment of principal			$ 556,000,000
Term Loan | Alight Holdings
Debt Instrument [Line Items]
Debt term						7 years
Repayments of principal to refinance debt				270,000,000
Term Loan, Amended
Debt Instrument [Line Items]
Payment terms											The Company is required to make principal payments at the end of each fiscal quarter based on defined terms in the agreement with the remaining principal balances due on the maturity dates.
Principal payment							$ 571,000,000			$ 25,000,000	$ 31,000,000
Term Loan, Amended | Alight Holdings
Debt Instrument [Line Items]
Debt balance				$ 1,986,000,000
Maturity Date				Oct. 31, 2026
Principal payment								$ 13,000,000
Term Loan, Amended | Minimum | Alight Holdings
Debt Instrument [Line Items]
Debt instrument, variable interest rate				0.50%
Term Loan, Third Incremental
Debt Instrument [Line Items]
Debt balance		$ 525,000,000
Maturity Date		Aug. 31, 2028
Term Loan, B-1 [Member]
Debt Instrument [Line Items]
Aggregate increase in debt liability	$ 65,000,000
Maturity Date									Aug. 31, 2028	Aug. 31, 2028
Term Loan, B-1 [Member] | SOFR | Minimum | Alight Holdings
Debt Instrument [Line Items]
Debt instrument, variable interest rate									2.75%
Term Loan, B-1 [Member] | SOFR | Maximum | Alight Holdings
Debt Instrument [Line Items]
Debt instrument, variable interest rate									3.00%
Secured Senior Notes
Debt Instrument [Line Items]
Maturity Date										Jun. 01, 2025
Payment terms											interest at a fixed rate of 5.75% per annum, payable semi-annually on June 1 and December 1 of each year
Secured Senior Notes | Alight Holdings
Debt Instrument [Line Items]
Maturity Date					Jun. 01, 2025
Face amount					$ 300,000,000
Interest rate					5.75%
Debt payment beginning date					Dec. 01, 2020
Fifth Incremental Term Loans
Debt Instrument [Line Items]
Debt balance									$ 2,507,000,000
Maturity Date										Aug. 31, 2028