v3.21.2

Commitments and Contingencies - Additional Information (Details) $ in Millions	9 Months Ended
	Sep. 30, 2021 USD ($)
Purchase Obligations
Commitment And Contingencies [Line Items]
Purchase obligation, remainder of 2021	$ 6
Purchase obligation, 2022	26
Purchase obligation, 2023	26
Purchase obligation, 2024	27
Purchase obligation, 2025	9
Purchase obligation, thereafter	7
Service Obligations
Commitment And Contingencies [Line Items]
Service obligation, remainder of 2021	34
Service obligation, 2022	141
Service obligation, 2023	147
Service obligation, 2024	154
Service obligation, 2025	162
Service obligation, thereafter	$ 502
Service obligation agreement termination fees percentage	25.00%
Service obligation maturity period	10 years

X

- Definition

Commitment and contingencies.

+ References

No definition available.

+ Details

Name:	alit_CommitmentAndContingenciesLineItems
Namespace Prefix:	alit_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Contractual obligation due after fourth year.

+ References

No definition available.

+ Details

Name:	alit_ContractualObligationDueAfterFourthYear
Namespace Prefix:	alit_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Purchase obligation due after fourth year.

+ References

No definition available.

+ Details

Name:	alit_PurchaseObligationDueAfterFourthYear
Namespace Prefix:	alit_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Service obligation agreement termination fees percentage.

+ References

No definition available.

+ Details

Name:	alit_ServiceObligationAgreementTerminationFeesPercentage
Namespace Prefix:	alit_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

X

- Definition

Service obligation maturity period.

+ References

No definition available.

+ Details

Name:	alit_ServiceObligationMaturityPeriod
Namespace Prefix:	alit_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of contractual obligation to be paid in fourth fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

Name:	us-gaap_ContractualObligationDueInFourthYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of contractual obligation to be paid in next fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

Name:	us-gaap_ContractualObligationDueInNextTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of contractual obligation to be paid in second fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

Name:	us-gaap_ContractualObligationDueInSecondYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of contractual obligation to be paid in third fiscal year following current fiscal year. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

Name:	us-gaap_ContractualObligationDueInThirdYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of contractual obligation to be paid in remainder of current fiscal year.

+ References

No definition available.

+ Details

Name:	us-gaap_ContractualObligationFutureMinimumPaymentsDueRemainderOfFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of purchase arrangement to be paid in fourth fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

Name:	us-gaap_PurchaseObligationDueInFourthYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of purchase arrangement to be paid in next fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

Name:	us-gaap_PurchaseObligationDueInNextTwelveMonths
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of purchase arrangement to be paid in second fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

Name:	us-gaap_PurchaseObligationDueInSecondYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of purchase arrangement to be paid in third fiscal year following current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment. Excludes interim and annual periods when interim periods are reported from current statement of financial position date (rolling approach).

+ References

No definition available.

+ Details

Name:	us-gaap_PurchaseObligationDueInThirdYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Amount of purchase arrangement to be paid in remainder of current fiscal year. Includes, but is not limited to, recorded and unrecorded purchase obligations, long-term purchase commitment, and short-term purchase commitment.

+ References

No definition available.

+ Details

Name:	us-gaap_PurchaseObligationFutureMinimumPaymentsRemainderOfFiscalYear
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

Name:	us-gaap_RegistrationPaymentArrangementByArrangementAxis=alit_PurchaseObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	us-gaap_RegistrationPaymentArrangementByArrangementAxis=alit_ServiceObligationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: