Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2021
Mar. 07, 2022
Jun. 30, 2021
Document Information [Line Items]      
Document Type 10-K    
Amendment Flag false    
Document Period End Date Dec. 31, 2021    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY    
Entity Registrant Name Alight, Inc. / Delaware    
Entity Central Index Key 0001809104    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Current Fiscal Year End Date --12-31    
Entity Filer Category Large Accelerated Filer    
Entity Public Float     $ 1,011,195,000
Entity Shell Company false    
Entity Small Business false    
Entity Emerging Growth Company false    
Title of 12(b) Security Class A Common Stock, par value $0.0001 per share    
Trading Symbol ALIT    
Security Exchange Name NYSE    
Entity File Number 001-39299    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 86-1849232    
Entity Address, Address Line One 4 Overlook Point    
Entity Address, City or Town Lincolnshire    
Entity Address, State or Province IL    
Entity Address, Postal Zip Code 60069    
City Area Code 224    
Local Phone Number 737-7000    
Document Annual Report true    
Document Transition Report false    
ICFR Auditor Attestation Flag false    
Auditor Name Ernst & Young LLP    
Auditor Location Chicago, Illinois    
Auditor Firm ID 42    
Documents Incorporated by Reference

Portions of the Registrant's definitive Proxy Statement for its 2022 annual meeting of stockholders to be filed with the Securities and Exchange Commission not later than 120 days after the end of the Company’s fiscal year are incorporated by reference into Part III, Items 10-14 of this Annual Report on Form 10-K.

    
Class A Common Stock      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   465,211,609  
Class B-1 Common Stock      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   4,990,453  
Class B-2 Common Stock      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   4,990,453  
Class V Common Stock      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   76,220,431  
Class Z-A Common Stock      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   5,046,819  
Class Z-B-1 Common Stock      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   274,379  
Class Z-B-2 Common Stock      
Document Information [Line Items]      
Entity Common Stock, Shares Outstanding   274,379