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Stock Options (Details) (USD $)
 3 Months Ended 6 Months Ended
Jun. 30, 2011
Jun. 30, 2010
Jun. 30, 2011
Jun. 30, 2010
Stock option details        
Stock-based compensation expense $ 93,000 $ 145,000 $ 195,000 $ 297,000
Stock option activity        
Options outstanding at the beginning of the period (in shares)     764,982  
Plus: Options granted (in shares)     1,500  
Less:        
Options exercised (in shares)     7,092  
Options expired (in shares)     81,218  
Options forfeited (in shares)     25,221  
Options outstanding at the end of the period (in shares) 652,951   652,951  
Options fully vested and exercisable at the end of the period (in shares) 323,472   323,472  
Stock options, weighted average exercise price        
Options outstanding at the beginning, weighted average exercise price (in dollars per share)     $ 20.65  
Plus: Options granted, weighted average exercise price (in dollars per share) $ 18.44   $ 18.44  
Less:        
Options exercised, weighted average exercise price (in dollars per share) $ 17.95   $ 17.95  
Options expired, weighted average exercise price (in dollars per share) $ 19.93   $ 19.93  
Options forfeited, weighted average exercise price (in dollars per share) $ 18.86   $ 18.86  
Options outstanding at the end, weighted average exercise price (in dollars per share) $ 20.86   $ 20.86  
Options fully vested and exercisable at the end, weighted average exercise price (in dollars per share) $ 24.73   $ 24.73  
Stock options, weighted average remaining contractual term (in years)        
Options outstanding at the end, weighted average remaining contractual term (in years) 3.77   3.77  
Options fully vested and exercisable at the end, weighted average remaining contractual term (in years)     2.82  
Stock options, aggregate intrinsic value        
Options outstanding at the end, aggregate intrinsic value 1,333,000   1,333,000  
Options fully vested and exercisable at the end, aggregate intrinsic value 199,000   199,000  
International bancshares corporation stock option plan (2005 Plan)
        
Stock option details        
Increase in the number of shares available for stock option grants under the 2005 plan (in shares)     300,000  
Maximum exercisable period for options granted (in years)     10 years  
Shares available for future grants (in shares) 210,322   210,322  
Stock-based compensation cost, unrecognized, related to non-vested options $ 480,000   $ 480,000  
Stock-based compensation cost, unrecognized, related to non-vested options, weighted-average period of recognition (in years)     1.3  
Incentive stock options granted to 10% shareholders
        
Stock option details        
Maximum exercisable period for options granted (in years)     5 years