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International Bancshares Corporation (Parent Company Only) Financial Information Statements of Income (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2020 | Dec. 31, 2019 | Dec. 31, 2018
Expenses: |  |  |
Interest expense (Debentures) | $ 3,832 | $ 6,435 | $ 6,989
Provision for credit loss | 45,379 | 18,843 | 6,112
Income before federal income taxes and equity in undistributed net income of subsidiaries | 211,758 | 259,954 | 272,583
Income tax expense | 44,439 | 54,850 | 56,652
Net income | 167,319 | 205,104 | 215,931
Parent Company [Member] | Reportable Legal Entities |  |  |
Income: |  |  |
Dividends from subsidiaries | 130,950 | 127,750 | 105,000
Interest income on notes receivable | 357 | 922 |
Interest income on other investments | (1,126) | (514) | 8,208
Other | 5 | 18 | 1,988
Total income | 130,186 | 128,176 | 115,196
Expenses: |  |  |
Interest expense (Debentures) | 3,832 | 6,435 | 6,989
Provision for credit loss | 27 |  |
Other | 1,988 | 2,749 | 2,930
Total expenses | 5,847 | 9,184 | 9,919
Income before federal income taxes and equity in undistributed net income of subsidiaries | 124,339 | 118,992 | 105,277
Income tax expense | (1,339) | (1,878) | 481
Income before equity in undistributed net income of subsidiaries | 125,678 | 120,870 | 104,796
Equity in undistributed net income of subsidiaries | 41,641 | 84,234 | 111,135
Net income | $ 167,319 | $ 205,104 | $ 215,931

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