Document and Entity Information - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
	Dec. 31, 2021	Jan. 26, 2022	Jun. 30, 2021	Dec. 31, 2020
Entity Information [Line Items]
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2021
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Transition Report	false
Entity File Number	001-13221
Entity Registrant Name	CULLEN/FROST BANKERS, INC.
Entity Incorporation, State or Country Code	TX
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
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 6,800
Entity Common Stock, Shares Outstanding		64,023,571
Common Stock, Par or Stated Value Per Share	$ 0.01			$ 0.01
Entity Central Index Key	0000039263
Amendment Flag	false
Document Fiscal Period Focus	FY
Auditor Name	Ernst & Young LLP
Auditor Location	San Antonio, Texas
Auditor Firm ID	42
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
Series B Preferred Stock [Member] | NEW YORK STOCK EXCHANGE, INC. [Member]
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