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Cover Page - USD ($)
 12 Months Ended
Dec. 31, 2020
Feb. 22, 2021
Jun. 30, 2020
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2020    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 0-25346    
Entity Registrant Name ACI WORLDWIDE, INC.    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 47-0772104    
Entity Address, Address Line One 3520 Kraft Rd,    
Entity Address, Address Line Two Suite 300    
Entity Address, City or Town Naples,    
Entity Address, State or Province FL    
Entity Address, Postal Zip Code 34105    
City Area Code 239    
Local Phone Number 403-4660    
Title of 12(b) Security Common Stock, $0.005 par value    
Trading Symbol ACIW    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 2,767,626,204
Entity Common Stock, Shares Outstanding   117,142,557  
Documents Incorporated by Reference Documents Incorporated by Reference – Portions of the registrant’s definitive Proxy Statement for the Annual Meeting of Shareholders to be held on June 2, 2021, are incorporated by reference in Part III of this report. This registrant’s Proxy Statement will be filed with the Securities and Exchange Commission pursuant to Regulation 14A.    
Amendment Flag false    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
Entity Central Index Key 0000935036