Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	May 04, 2020
Document and Entity Information
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Mar. 31, 2020
Document Transition Report	false
Entity File Number	000-09439
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
Title of 12(b) Security	Common Stock
Trading Symbol	IBOC
Security Exchange Name	NASDAQ
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 Common Stock, Shares Outstanding		63,270,747
Entity Central Index Key	0000315709
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Amendment Flag	false