| Document and Entity Information Document | Sep. 07, 2021 | Jul. 29, 2021 |
| Entity Information [Line Items] | ||
| Document Type | 8-K | |
| Document Period End Date | Sep. 7, 2021 | |
| Entity Registrant Name | Cullen/Frost Bankers, Inc. | |
| Entity Incorporation, State or Country Code | TX | |
| Entity File Number | 001-13221 | |
| Entity Tax Identification Number | 74-1751768 | |
| Entity Address, Address Line One | 111 W. Houston Street, | |
| Entity Address, City or Town | San Antonio, | |
| Entity Address, State or Province | TX | |
| Entity Address, Postal Zip Code | 78205 | |
| City Area Code | (210) | |
| 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 | |
| Amendment Flag | false | |
| Entity Central Index Key | 0000039263 | |
| Common Stock | NEW YORK STOCK EXCHANGE, INC. | ||
| Entity Information [Line Items] | ||
| Title of 12(b) Security | Common Stock, $.01 Par Value | |
| Trading Symbol | CFR | |
| Security Exchange Name | NYSE | |
| Series B Preferred Stock | NEW YORK STOCK EXCHANGE, INC. | ||
| Entity 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 |
