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Stock Options (Details) (USD $)
3 Months Ended9 Months Ended
Sep. 30, 2011
year
Sep. 30, 2010
Sep. 30, 2011
year
segment
entity
day
trust
semiannual
quarter
Sep. 30, 2010
Stock option details    
Stock-based compensation expense$ 85,000$ 135,000$ 280,000$ 431,000
Stock option activity    
Options outstanding at the beginning of the period (in shares)  764,982 
Plus: Options granted (in shares)  3,000 
Less:    
Options exercised (in shares)  7,092 
Options expired (in shares)  81,218 
Options forfeited (in shares)  46,797 
Options outstanding at the end of the period (in shares)632,875 632,875 
Options fully vested and exercisable at the end of the period (in shares)320,640 320,640 
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)$ 16.06 $ 16.06 
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)$ 21.41 $ 21.41 
Options outstanding at the end, weighted average exercise price (in dollars per share)$ 20.72 $ 20.72 
Options fully vested and exercisable at the end, weighted average exercise price (in dollars per share)$ 24.54 $ 24.54 
Stock options, weighted average remaining contractual term (in years)    
Options outstanding at the end, weighted average remaining contractual term (in years)3.56 3.56 
Options fully vested and exercisable at the end, weighted average remaining contractual term (in years)  2.64 
Stock options, aggregate intrinsic value    
Options outstanding at the end, aggregate intrinsic value568,000 568,000 
Options fully vested and exercisable at the end, aggregate intrinsic value85,000 85,000 
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)230,398 230,398 
Stock-based compensation cost, unrecognized, related to non-vested options$ 402,000 $ 402,000 
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