| Document and Entity Information | May 07, 2020 |
| Document and Entity Information | |
| Document Type | 8-K |
| Document Period End Date | Aug. 6, 2020 |
| Entity Registrant Name | INTERNATIONAL BANCSHARES CORPORATION |
| Entity Incorporation, State or Country Code | TX |
| Entity File Number | 000-9439 |
| Entity Tax Identification Number | 74-2157138 |
| Entity Address, Address Line One | 1200 San Bernardo |
| Entity Address, City or Town | Laredo |
| Entity Address, State or Province | TX |
| Entity Address, Postal Zip Code | 78040-1359 |
| City Area Code | 956 |
| Local Phone Number | 722-7611 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | IBOC |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000315709 |
| Amendment Flag | false |
