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Cover Document
Apr. 28, 2021
Apr. 29, 2020
Document Information [Line Items]    
Amendment Flag   false
City Area Code (210)  
Entity Address, Address Line One 111 W. Houston Street,  
Entity Incorporation, State or Country Code TX  
Document Type 8-K  
Document Period End Date Apr. 28, 2021  
Entity Registrant Name Cullen/Frost Bankers, Inc.  
Entity File Number 001-13221  
Entity Tax Identification Number 74-1751768  
Entity Address, City or Town San Antonio,  
Entity Address, State or Province TX  
Entity Address, Postal Zip Code 78205  
Local Phone Number 220-4011  
Written Communications false  
Soliciting Material false  
Pre-commencement Tender Offer false  
Pre-commencement Issuer Tender Offer false  
Entity Emerging Growth Company false  
Entity Central Index Key   0000039263
Series B Preferred Stock | NEW YORK STOCK EXCHANGE, INC.    
Document Information [Line Items]    
Title of 12(b) Security Depositary Shares, each representing a 1/40th interest in a share of 4.450% Non-Cumulative Perpetual Preferred Stock, Series B  
Trading Symbol CFR.PrB  
Security Exchange Name NYSE  
Common Stock | NEW YORK STOCK EXCHANGE, INC.    
Document Information [Line Items]    
Title of 12(b) Security Common Stock, $.01 Par Value  
Trading Symbol CFR  
Security Exchange Name NYSE