Borrowed Funds (Narrative) (Detail) $ in Thousands		12 Months Ended
	Mar. 17, 2017 USD ($)	Dec. 31, 2025 USD ($) quarter	Dec. 31, 2024 USD ($)	Dec. 31, 2004 USD ($)
Borrowed Funds [Line Items]
Federal funds purchased		$ 18,775	$ 21,975
Repurchase agreements		4,525,855	4,342,941
Subordinated notes		99,804	99,648
Junior subordinated deferrable interest debentures		$ 123,242	123,184
Maximum number of quarterly periods by which the corporation has the right to defer interest payments on junior subordinated deferrable interest debentures | quarter		20
Cullen Frost Capital Trust II [Member]
Borrowed Funds [Line Items]
Maturity date				Mar. 01, 2034
Proceeds from Issuance of Trust Preferred Securities				$ 120,000
Floating rate 3 month SOFR plus margin				1.81%
Proceeds from issuance of Common Equity Securities				$ 3,700
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 Loaned or Sold under Agreements to Repurchase [Member] | Minimum [Member]
Borrowed Funds [Line Items]
Short term debt maturity period		30 days
Securities Loaned or Sold under Agreements to Repurchase [Member] | Maximum [Member]
Borrowed Funds [Line Items]
Short term debt maturity period		90 days
Subordinated Debt [Member]
Borrowed Funds [Line Items]
Subordinated notes	$ 100,000
Subordinated notes interest rate	4.50%
Maturity date	Mar. 17, 2027
Unamortized debt issuance cost related to note		$ 196	352
Junior Subordinated Deferrable Interest Debenture [Member] | Cullen Frost Capital Trust II [Member]
Borrowed Funds [Line Items]
Unamortized debt issuance cost related to note		470	528
Junior subordinated deferrable interest debentures		$ 123,700	$ 123,700	$ 123,700
Floating rate 3 month SOFR plus margin		5.60%	6.28%