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Borrowed Funds (Narrative) (Detail) (USD $)
12 Months Ended 1 Months Ended
Dec. 31, 2014
quarter
Dec. 31, 2013
Dec. 31, 2004
Feb. 28, 2007
Borrowed Funds [Line Items]        
Federal funds purchased $ 12,000,000us-gaap_FederalFundsPurchased $ 200,000us-gaap_FederalFundsPurchased    
Securities sold under agreements to repurchase 791,100,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase 668,100,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase    
Maximum number of quarterly periods by which the corporation has the right to defer interest payments on junior subordinated deferrable interest debentures 20cfr_Maximumnumberofquarterlyperiodsbywhichthecorporationhastherighttodeferinterestpaymentsonjuniorsubordinateddeferrableinterestdebentures      
Junior subordinated deferrable interest debentures 137,115,000us-gaap_JuniorSubordinatedNotes 123,712,000us-gaap_JuniorSubordinatedNotes    
Common equity securities   144,486,000us-gaap_StockIssuedDuringPeriodValueNewIssues    
Maximum allowable percentage of core capital included in Tier 1 Capital by Most Bank Holding Companies 25.00%cfr_MaximumAllowablePercentageOfCoreCapitalIncludedInTierOneCapitalByMostBankHoldingCompanies      
Proceeds from Issuance of Trust Preferred Securities 133,000,000us-gaap_ProceedsFromIssuanceOfTrustPreferredSecurities 120,000,000us-gaap_ProceedsFromIssuanceOfTrustPreferredSecurities    
Trust II [Member]        
Borrowed Funds [Line Items]        
Advances, variable-rate 1.78%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= cfr_CullenFrostCapitalTrustIIMember
1.79%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= cfr_CullenFrostCapitalTrustIIMember
1.55%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= cfr_CullenFrostCapitalTrustIIMember
 
LIBOR plus margin     Three-month LIBOR plus a margin of 1.55%  
Trust preferred securities, issued     120,000,000cfr_TrustPreferredSecuritiesIssued
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= cfr_CullenFrostCapitalTrustIIMember
 
Maturity date of trust preferred securities March 1, 2034      
WNB Capital Trust I [Member]        
Borrowed Funds [Line Items]        
Advances, variable-rate 2.58%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= cfr_WNBCapitalTrustIMember
  2.35%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= cfr_WNBCapitalTrustIMember
 
Trust preferred securities, issued     13,000,000cfr_TrustPreferredSecuritiesIssued
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= cfr_WNBCapitalTrustIMember
 
Maturity date of trust preferred securities July 23, 2034      
Federal Funds Purchased [Member] | Minimum [Member]        
Borrowed Funds [Line Items]        
Short term debt maturity period 1 day      
Federal Funds Purchased [Member] | Maximum [Member]        
Borrowed Funds [Line Items]        
Short term debt maturity period 90 days      
Securities Sold under Agreements to Repurchase [Member] | Minimum [Member]        
Borrowed Funds [Line Items]        
Short term debt maturity period 30 days      
Securities Sold under Agreements to Repurchase [Member] | Maximum [Member]        
Borrowed Funds [Line Items]        
Short term debt maturity period 90 days      
Subordinated Notes Payable [Member]        
Borrowed Funds [Line Items]        
Subordinated Notes       100,000,000us-gaap_SubordinatedDebt
/ us-gaap_LongtermDebtTypeAxis
= cfr_SubordinatedNotesPayableMember
Subordinated Notes interest rate       5.75%us-gaap_SubordinatedBorrowingInterestRate
/ us-gaap_LongtermDebtTypeAxis
= cfr_SubordinatedNotesPayableMember
Maturity date       Feb. 15, 2017
Advances, variable-rate 0.76%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_LongtermDebtTypeAxis
= cfr_SubordinatedNotesPayableMember
0.77%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_LongtermDebtTypeAxis
= cfr_SubordinatedNotesPayableMember
  0.53%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_LongtermDebtTypeAxis
= cfr_SubordinatedNotesPayableMember
LIBOR plus margin       Three-month LIBOR for the related interest period plus 0.53%
Percentage of redemption price 100.00%cfr_PercentageOfRedemptionPriceNotesPayable
/ us-gaap_LongtermDebtTypeAxis
= cfr_SubordinatedNotesPayableMember
     
Unamortized debt issuance cost related to note 250,000us-gaap_UnamortizedDebtIssuanceExpense
/ us-gaap_LongtermDebtTypeAxis
= cfr_SubordinatedNotesPayableMember
370,000us-gaap_UnamortizedDebtIssuanceExpense
/ us-gaap_LongtermDebtTypeAxis
= cfr_SubordinatedNotesPayableMember
   
Junior Subordinated Deferrable Interest Debenture [Member] | Trust II [Member]        
Borrowed Funds [Line Items]        
Unamortized debt issuance cost related to note 1,100,000us-gaap_UnamortizedDebtIssuanceExpense
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorSubordinatedDebtMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= cfr_CullenFrostCapitalTrustIIMember
1,200,000us-gaap_UnamortizedDebtIssuanceExpense
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorSubordinatedDebtMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= cfr_CullenFrostCapitalTrustIIMember
   
Junior subordinated deferrable interest debentures 123,712,000.000us-gaap_JuniorSubordinatedNotes
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorSubordinatedDebtMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= cfr_CullenFrostCapitalTrustIIMember
123,712,000us-gaap_JuniorSubordinatedNotes
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorSubordinatedDebtMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= cfr_CullenFrostCapitalTrustIIMember
   
Common equity securities 3,700,000.0us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorSubordinatedDebtMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= cfr_CullenFrostCapitalTrustIIMember
     
Junior Subordinated Deferrable Interest Debenture [Member] | WNB Capital Trust I [Member]        
Borrowed Funds [Line Items]        
Junior subordinated deferrable interest debentures 13,403,000.000us-gaap_JuniorSubordinatedNotes
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorSubordinatedDebtMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= cfr_WNBCapitalTrustIMember
     
Common equity securities $ 403,000us-gaap_StockIssuedDuringPeriodValueNewIssues
/ us-gaap_LongtermDebtTypeAxis
= us-gaap_JuniorSubordinatedDebtMember
/ us-gaap_VariableInterestEntitiesByClassificationOfEntityAxis
= cfr_WNBCapitalTrustIMember