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Document and Entity Information - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2019
Feb. 24, 2020
Jun. 30, 2019
Document and Entity Information      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2019    
Document Transition Report false    
Entity File Number 0-09439    
Entity Registrant Name INTERNATIONAL BANCSHARES CORPORATION    
Entity Incorporation, State or Country Code TX    
Entity Tax Identification Number 74-2157138    
Entity Address, Address Line One 1200 San Bernardo Avenue    
Entity Address, City or Town Laredo    
Entity Address, State or Province TX    
Entity Address, Postal Zip Code 78042 - 1359    
City Area Code 956    
Local Phone Number 722-7611    
Entity Well-known Seasoned Issuer Yes    
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    
Entity Shell Company false    
Entity Public Float     $ 2,475,805
Entity Common Stock, Shares Outstanding   65,207,831  
Entity Central Index Key 0000315709    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2019    
Document Fiscal Period Focus FY    
Amendment Flag false