Debt - Additional Information (Details) - USD ($)		1 Months Ended								3 Months Ended		9 Months Ended
		Sep. 30, 2023	Mar. 31, 2023	Jan. 31, 2022	Aug. 31, 2021	Jul. 31, 2021	Aug. 31, 2020	May 31, 2020	May 31, 2017	Mar. 31, 2024	Mar. 31, 2023	Sep. 30, 2023	Dec. 31, 2023
Debt Instrument [Line Items]
Debt balance										$ 2,787,000,000			$ 2,794,000,000
Amortization of financing fees and benefits										(1,000,000)	$ (1,000,000)
Interest expense related to debt instruments										55,000,000	51,000,000
Interest Expense
Debt Instrument [Line Items]
Amortization of financing fees and benefits										$ 1,000,000	1,000,000
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
Amended Revolving Credit Facility
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										$ 3,000,000
Proceeds from Lines of Credit												$ 0
Amended Revolving Credit Facility | 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]
Repayment of principal						$ 556,000,000
Term Loan | Alight Holdings
Debt Instrument [Line Items]
Debt term			7 years						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										$ 6,000,000	$ 6,000,000
Swap agreement expiration period		2026-12
Term Loan, Amended | Alight Holdings
Debt Instrument [Line Items]
Debt balance							$ 1,986,000,000
Maturity Date							Oct. 31, 2026
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	Aug. 31, 2028
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										$ 2,507,000,000
Maturity Date	[1]									Aug. 31, 2028
Term Loan, B-1
Debt Instrument [Line Items]
Maturity Date		Aug. 31, 2028
Increase in term loan			$ 65,000,000
Term Loan, B-1 | SOFR | Minimum | Alight Holdings
Debt Instrument [Line Items]
Debt instrument, variable interest rate		2.75%
Term Loan, B-1 | SOFR | Maximum | Alight Holdings
Debt Instrument [Line Items]
Debt instrument, variable interest rate		3.00%

[1] The net balance for the Fifth Incremental Term Loans includes unamortized debt issuance costs at March 31, 2024 and December 31, 2023 of approximately $8 million and $8 million, respectively.