Financial Instruments - Seller Earnouts - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2022	Dec. 31, 2021
Financial Instruments [Abstract]
Balance portion of seller earnouts	$ 1
Fair value of seller earnouts	121	$ 135
Loss (gain) from change in fair value of seller earnouts	$ (14)

The seller earnout fair value of shares that would be issued, determined under the conditions specified in the contract if the settlement were to occur at the reporting date.

No definition available.

Name:	alit_FinancialInstrumentsSubjectToMandatoryRedemptionSettlementTermsSellerEarnOutFairValueOfShares
Namespace Prefix:	alit_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration