v3.21.2

Basis of Presentation and Nature of Business - Additional Information (Details) |  | 9 Months Ended
--- | --- | ---
Jul. 02, 2021 | Sep. 30, 2021
Basis Of Presentation And Nature Of Business [Line Items] |  |
Date of incorporation |  | Mar. 26, 2020
Foley Trasimene Acquisition Corp |  |
Basis Of Presentation And Nature Of Business [Line Items] |  |
Business combination, closing date of acquisition | Jul. 02, 2021 |
Foley Trasimene Acquisition Corp | Alight Holdings |  |
Basis Of Presentation And Nature Of Business [Line Items] |  |
Non-voting ownership percentage held by noncontrolling interest | 15.00% |
Alight | Foley Trasimene Acquisition Corp | Alight Holdings |  |
Basis Of Presentation And Nature Of Business [Line Items] |  |
Percentage of economic interest | 85.00% |
Business combination, percentage of voting power | 100.00% |

X
- Definition Basis of presentation and nature of business. + References No definition available. + Details
Name: | alit_BasisOfPresentationAndNatureOfBusinessLineItems
Namespace Prefix: | alit_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Date when an entity was incorporated + References No definition available. + Details
Name: | dei_EntityIncorporationDateOfIncorporation
Namespace Prefix: | dei_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Date when the acquirer obtains control of the acquiree, in CCYY-MM-DD format. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=116868678&loc=d3e848-128460 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 25 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=116868678&loc=d3e845-128460 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 + Details
Name: | us-gaap_BusinessAcquisitionEffectiveDateOfAcquisition1
Namespace Prefix: | us-gaap_
Data Type: | xbrli:dateItemType
Balance Type: | na
Period Type: | duration

X
- Definition Percentage of voting equity interests acquired at the acquisition date in the business combination. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=79982066&loc=d3e1392-128463 + Details
Name: | us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The equity interest of noncontrolling shareholders, partners or other equity holders in consolidated entity. + References No definition available. + Details
Name: | us-gaap_MinorityInterestOwnershipPercentageByNoncontrollingOwners
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Definition The parent entity's interest in net assets of the subsidiary, expressed as a percentage. + References No definition available. + Details
Name: | us-gaap_MinorityInterestOwnershipPercentageByParent
Namespace Prefix: | us-gaap_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | instant

X
- Details
Name: | us-gaap_BusinessAcquisitionAxis=alit_FoleyTrasimeneAcquisitionCorpMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_OwnershipAxis=alit_AlightHoldingsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_ConsolidatedEntitiesAxis=alit_AlightGroupInc.Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |