| Document and Entity Information | Jul. 22, 2019 |
| Entity Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Jul. 22, 2019 |
| 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 [Member] | NEW YORK STOCK EXCHANGE, INC. [Member] | |
| Entity Information [Line Items] | |
| Title of 12(b) Security | Common Stock, $.01 Par Value |
| Trading Symbol | CFR |
| Security Exchange Name | NYSE |
| Preferred Class A [Member] | NEW YORK STOCK EXCHANGE, INC. [Member] | |
| Entity Information [Line Items] | |
| Title of 12(b) Security | 5.375% Non-Cumulative Perpetual Preferred Stock, Series A |
| Trading Symbol | CFR.PRA |
| Security Exchange Name | NYSE |
| Former Address [Member] | |
| Entity Information [Line Items] | |
| Entity Address, Address Line One | 100 W. Houston Street, |
| Entity Address, City or Town | San Antonio, |
| Entity Address, State or Province | TX |
| Entity Address, Postal Zip Code | 78205 |
