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Borrowed Funds (Narrative) (Detail) $ in Thousands |  | 3 Months Ended | 12 Months Ended
--- | --- | --- | ---
Mar. 17, 2017 USD ($) | Dec. 31, 2022 USD ($) | Dec. 31, 2023 USD ($) quarter | Dec. 31, 2022 USD ($) | Dec. 31, 2021 USD ($) | Dec. 31, 2004 USD ($)
Borrowed Funds [Line Items] |  |  |  |  |  |
Federal funds purchased |  | $ 51,650 | $ 14,200 | $ 51,650 |  |
Repurchase agreements |  | 4,660,641 | 4,127,188 | 4,660,641 |  |
Subordinated notes |  | 99,335 | 99,491 | 99,335 |  |
Junior subordinated deferrable interest debentures |  | 123,069 | 123,127 | 123,069 |  |
Principal payments on long-term borrowings |  |  | $ 0 | 0 | $ 13,403 |
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
Proceeds from issuance of Common Equity Securities |  |  |  |  |  | $ 3,700
Cullen Frost Capital Trust II [Member] | SOFR |  |  |  |  |  |
Borrowed Funds [Line Items] |  |  |  |  |  |
Floating rate 3 month Libor plus margin |  |  |  |  |  | 1.81%
Cullen Frost Capital Trust II [Member] | LIBOR [Member] |  |  |  |  |  |
Borrowed Funds [Line Items] |  |  |  |  |  |
Floating rate 3 month Libor plus margin |  |  |  |  |  | 1.55%
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 |  | 665 | $ 509 | 665 |  |
Junior Subordinated Deferrable Interest Debenture [Member] | Cullen Frost Capital Trust II [Member] |  |  |  |  |  |
Borrowed Funds [Line Items] |  |  |  |  |  |
Unamortized debt issuance cost related to note |  | 643 | 585 | 643 |  |
Junior subordinated deferrable interest debentures |  | 123,700 | $ 123,700 | $ 123,700 |  | $ 123,700
Junior Subordinated Deferrable Interest Debenture [Member] | Cullen Frost Capital Trust II [Member] | SOFR |  |  |  |  |  |
Borrowed Funds [Line Items] |  |  |  |  |  |
Floating rate 3 month Libor plus margin |  |  | 7.22% |  |  |
Junior Subordinated Deferrable Interest Debenture [Member] | Cullen Frost Capital Trust II [Member] | LIBOR [Member] |  |  |  |  |  |
Borrowed Funds [Line Items] |  |  |  |  |  |
Floating rate 3 month Libor plus margin |  |  |  | 6.31% |  |
Junior Subordinated Deferrable Interest Debenture [Member] | WNB Capital Trust I [Member] |  |  |  |  |  |
Borrowed Funds [Line Items] |  |  |  |  |  |
Principal payments on long-term borrowings |  | $ 13,400 |  |  |  |

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