Financial Instruments - Seller Earnouts - Additional Information (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2021
Financial Instruments [Abstract]
Fair value of seller earnouts		$ 144
Loss (gain) from change in fair value of seller earnouts	$ 35

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