Consolidated Statements of Income (Parenthetical) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Consolidated Statements of Income
Impairment charges, other-than-temporary impairment charges	$ 0	$ 0	$ 793
Impairment charges, other comprehensive income	$ 0	$ 0	$ 1,147

The amount of other than temporary impairment (OTTI) losses on equity securities, OTTI related to credit losses on debt securities, and OTTI losses on debt securities when the entity intends to sell the securities or it is more likely than not that the entity will be required to sell the securities before recovery of the amortized cost basis. Additionally, this item includes OTTI losses recognized during the period on investments accounted for under the cost method of accounting.

No definition available.

Name:	iboc_OtherThanTemporaryImpairmentLossesInvestmentPortionRecognizedInEarningsNet
Namespace Prefix:	iboc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration