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