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Debt - Additional Information (Details) - USD ($)				1 Months Ended						3 Months Ended		9 Months Ended
Debt - Additional Information (Details) - USD ($)	May 31, 2024	Aug. 31, 2023	Jul. 02, 2021	Jul. 31, 2024	Jun. 30, 2024	Sep. 30, 2023	Mar. 31, 2023	Aug. 31, 2020	May 31, 2017	Sep. 30, 2024	Sep. 30, 2023	Sep. 30, 2024	Sep. 30, 2023	Dec. 31, 2023	Aug. 31, 2021	May 31, 2020
Debt Instrument [Line Items]
Debt balance										$ 2,031,000,000		$ 2,031,000,000		$ 2,794,000,000
Interest expense related to debt instruments										40,000,000	$ 57,000,000	149,000,000	$ 163,000,000
Amortization of financing fees and benefits												(1,000,000)	(2,000,000)
Interest Expense
Debt Instrument [Line Items]
Amortization of financing fees and benefits										2,000,000	0	3,000,000	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
Line of credit, maximum borrowing capacity									$ 250,000,000
Amended Revolving Credit Facility
Debt Instrument [Line Items]
Debt balance										0		0		0
Line of credit, maximum borrowing capacity										300,000,000		300,000,000			$ 294,000,000
Unused letters of credit										0		0
Revolving credit facility, borrowings												0
Amended Revolving Credit Facility \| Alight Holdings
Debt Instrument [Line Items]
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]
Principal payment										6,000,000	6,000,000	19,000,000	19,000,000
Term Loan, Amended \| Alight Holdings
Debt Instrument [Line Items]
Debt balance								$ 1,986,000,000
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
Term Loan, B-1
Debt Instrument [Line Items]
Increase in term loan						$ 65,000,000
Term Loan, B-1 \| SOFR \| Alight Holdings
Debt Instrument [Line Items]
Debt instrument, variable interest rate		3.00%				2.75%
Fifth Incremental Term Loans
Debt Instrument [Line Items]
Debt balance						$ 2,507,000,000				0	$ 2,507,000,000	0	$ 2,507,000,000	2,488,000,000
Fifth Incremental Term Loans \| SOFR \| Alight Holdings
Debt Instrument [Line Items]
Debt instrument, variable interest rate	2.75%
Sixth Incremental Term Loans
Debt Instrument [Line Items]
Debt balance					$ 2,489,000,000					2,031,000,000		2,031,000,000		0
Repayments of debt				$ 440,000,000
Gain (loss) on extinguishment of debt				(1,000,000)
Sixth Incremental Term Loans \| SOFR \| Alight Holdings
Debt Instrument [Line Items]
Debt instrument, variable interest rate					2.25%
Secured Senior Notes
Debt Instrument [Line Items]
Debt balance										$ 0		$ 0		$ 306,000,000
Repayments of debt				300,000,000
Gain (loss) on extinguishment of debt				$ 4,000,000
Secured Senior Notes \| Alight Holdings
Debt Instrument [Line Items]
Face amount																$ 300,000,000
Interest rate																5.75%