v3.21.2

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 |

X
- Definition Financial instruments. + References No definition available. + Details
Name: | alit_FinancialInstrumentsAbstract
Namespace Prefix: | alit_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition 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. + References No definition available. + Details
Name: | alit_FinancialInstrumentsSubjectToMandatoryRedemptionSettlementTermsSellerEarnOutFairValueOfShares
Namespace Prefix: | alit_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

X
- Definition Loss (gain) from change in fair value of seller earnouts. + References No definition available. + Details
Name: | alit_LossGainFromChangeInFairValueOfSellerEarnouts
Namespace Prefix: | alit_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration