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Capital and Regulatory Matters - Additional Information (Detail)
$ in Thousands
 1 Months Ended 9 Months Ended 12 Months Ended
Feb. 15, 2016
USD ($)
Jan. 01, 2016
Feb. 28, 2007
Sep. 30, 2015
USD ($)
quarter
shares
Sep. 30, 2014
USD ($)
shares
Dec. 31, 2014
USD ($)
Jan. 01, 2019
Apr. 30, 2015
USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]                
Stock Repurchase Program, Authorized Amount               $ 100,000
Stock Repurchase Program, Period in Force       2 years        
Treasury Stock, Shares, Acquired | shares       1,095,308 0      
Purchase of treasury stock (1,095,308 shares in 2015)       $ (74,947) $ 0      
Preferred stock value issued include for regulatory tier one capital calculations       $ 144,500        
Preferred stock dividend rate percentage       5.375%        
Trust preferred securities       $ 133,000        
Corporation's aggregate subordinated notes       137,115   $ 137,115    
Maximum dividends available without prior regulatory approval       $ 399,400        
Maximum number of quarterly periods by which the corporation has the right to defer interest payments on junior subordinated deferrable interest debentures | quarter       20        
Two Thousand And Sixteen [Member] | Scenario, Forecast [Member]                
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]                
Tier One Capital Conservation Buffer To Risk Weighted Assets Required Under Regulatory   0.625%            
Tier One Risk Based Capital Conservation Buffer Initial Requirements Phasing Duration   4 years            
Two Thousand And Nineteen [Member] | Scenario, Forecast [Member]                
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]                
Minimum ratio of Tier 1 capital to risk-weighted assets             6.00%  
Minimum ratio of Total capital to risk-weighted assets             8.00%  
Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets             4.50%  
Capital Conservation Buffer             2.50%  
Two Thousand And Nineteen [Member] | Scenario, Forecast [Member] | Minimum [Member]                
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]                
Effective Tier 1 Capital Ratio, Upon Full Implementation             8.50%  
Tier One Risk Based Capital to Risk Weighted Assets, Ratio Upon Full Implementation             10.50%  
Minimum leverage ratio             4.00%  
Effective Common Equity Tier One Capital To Risk Weighted Assets, Ratio Upon Full Implementation             7.00%  
Trust Preferred Securities Tier One Allowable Portion [Member]                
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]                
Trust preferred securities       $ 33,300        
Trust Preferred Securities Tier Two Allowable Portion [Member]                
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]                
Trust preferred securities       99,800        
Floating Rate Subordinated Notes [Member]                
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]                
Corporation's aggregate subordinated notes       $ 100,000        
Per Year Percentage decrease in permissible portion of subordinate note included in Total Capital       20.00%        
Final years of the term of the notes       5 years        
Debt Instrument, Maturity Date     Feb. 15, 2017          
Permissible portion of subordinated note included in total capital       $ 20,000   $ 40,000    
Floating Rate Subordinated Notes [Member] | Two Thousand And Sixteen [Member] | Scenario, Forecast [Member]                
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]                
Permissible portion of subordinated note included in total capital $ 0              
Cullen/Frost [Member]                
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]                
Minimum ratio of Tier 1 capital to risk-weighted assets           4.00%    
Minimum ratio of Total capital to risk-weighted assets       13.96%   14.55%    
Minimum leverage ratio       7.91%   8.16%    
Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets       4.50%