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Capital and Regulatory Matters (Narrative) (Detail) |  |  |  |  | 12 Months Ended |
--- | --- | --- | --- | --- | --- | ---
Jan. 24, 2024 USD ($) | Jan. 25, 2023 USD ($) | Nov. 19, 2020 USD ($) $ / shares shares | Mar. 17, 2017 USD ($) | Dec. 31, 2023 USD ($) quarter $ / shares Rate shares | Dec. 31, 2022 USD ($) $ / shares Rate shares | Dec. 31, 2021 USD ($) shares | Jan. 01, 2019 Rate
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Credit loss optional transitional period under CECL |  |  |  |  | 5 years |  |  |
CECL Transition Amount |  |  |  |  | $ 30,800,000 | $ 46,200,000 |  |
Subordinated notes |  |  |  |  | $ 99,491,000 | $ 99,335,000 |  |
Treasury stock, shares, acquired | shares |  |  |  |  | 436,765 | 0 | 0 |
Treasury Stock, Value, Acquired, Cost Method |  |  |  |  | $ (42,720,000) | $ (4,391,000) | $ (3,864,000) |
Excise Tax Percentage | Rate |  |  |  |  | 1.00% |  |  |
Aggregate dividends available for payment without prior regulatory approval |  |  |  |  | $ 1,100,000,000 |  |  |
Maximum number of quarterly periods by which the corporation has the right to defer interest payments on junior subordinated deferrable interest debentures | quarter |  |  |  |  | 20 |  |  |
Series B Preferred Stock [Member] |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Non-cumulative perpetual preferred stock, dividend rate percentage |  |  | 4.45% |  |  |  |  |
Preferred Stock, Shares Issued | shares |  |  | 150,000 |  | 150,000 | 150,000 |  |
Preferred Stock, Aggregate Liquidation Preference, Value |  |  | $ 150,000,000 |  | $ 1,000 | $ 1,000 |  |
Preferred Stock, Par or Stated Value Per Share | $ / shares |  |  | $ 0.01 |  | $ 0.01 | $ 0.01 |  |
Liquidation preference per share | $ / shares |  |  | $ 1,000 |  |  |  |  |
Number of Preferred Stock equivalent depositary shares | shares |  |  | 40 |  |  |  |  |
Payments of Stock Issuance Costs |  |  | $ 4,500,000 |  |  |  |  |
Proceeds from issuance of preferred stock |  |  | $ 145,500,000 |  |  |  |  |
Series B Preferred Stock Equivalent Depositary Shares Member [Member] |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Liquidation preference per share | $ / shares |  |  | $ 25 |  |  |  |  |
Subordinated Debt [Member] |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Subordinated notes |  |  |  | $ 100,000,000 |  |  |  |
Subordinated notes interest rate |  |  |  | 4.50% |  |  |  |
Per year percentage of decreases in permissible portion of Subordinate Note included in Total Capital |  |  |  |  | 20.00% |  |  |
Final years of the term of the notes |  |  |  |  | 5 years |  |  |
Cullen/Frost [Member] |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Additional Tier 1 Capital beyond Common Equity Tier 1 |  |  |  |  | $ 145,500,000 | $ 145,500,000 |  |
Non-cumulative perpetual preferred stock, dividend rate percentage |  |  | 4.45% |  |  |  |  |
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio |  |  |  |  |  |  |  | 4.50%
Capital Conservation Buffer |  |  |  |  |  |  |  | 2.50%
Effective common equity tier one capital to risk weighted assets, ratio upon full implementation |  |  |  |  | 7.00% | 7.00% |  | 7.00%
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets | Rate |  |  |  |  |  |  |  | 6.00%
Effective tier one capital ratio, upon full implementation |  |  |  |  | 8.50% | 8.50% |  | 8.50%
Minimum ratio of Total capital to risk weighted assets | Rate |  |  |  |  | 15.18% | 14.84% |  | 8.00%
Tier one risk based capital to risk weighted assets, ratio upon full implementation |  |  |  |  | 10.50% | 10.50% |  | 10.50%
Minimum leverage ratio |  |  |  |  | 4.00% | 4.00% |  | 4.00%
Cullen/Frost [Member] | Trust Preferred Securities Tier Two Capital Allowable Portion [Member] |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Trust preferred securities |  |  |  |  | $ 120,000,000 | $ 120,000,000 |  |
Cullen/Frost [Member] | Subordinated Debt [Member] |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Permissible Portion Of Subordinated Note Included In Capital |  |  |  |  | 60,000,000 | 80,000,000 |  |
Frost Bank [Member] |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Additional Tier 1 Capital beyond Common Equity Tier 1 |  |  |  |  | $ 0 | $ 0 |  |
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio |  |  |  |  |  |  |  | 4.50%
Capital Conservation Buffer |  |  |  |  |  |  |  | 2.50%
Effective common equity tier one capital to risk weighted assets, ratio upon full implementation |  |  |  |  | 7.00% | 7.00% |  | 7.00%
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets | Rate |  |  |  |  |  |  |  | 6.00%
Effective tier one capital ratio, upon full implementation |  |  |  |  | 8.50% | 8.50% |  | 8.50%
Minimum ratio of Total capital to risk weighted assets | Rate |  |  |  |  | 14.20% | 13.80% |  | 8.00%
Tier one risk based capital to risk weighted assets, ratio upon full implementation |  |  |  |  | 10.50% | 10.50% |  | 10.50%
Minimum leverage ratio |  |  |  |  | 4.00% | 4.00% |  | 4.00%
stock repurchase 2024 plan | Subsequent Event |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Share repurchase authorized amount | $ 150,000,000 |  |  |  |  |  |  |
Stock repurchase program, period in force | 1 year |  |  |  |  |  |  |
stock repurchase 2023 plan |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Share repurchase authorized amount |  | $ 100,000,000 |  |  |  |  |  |
Stock repurchase program, period in force |  | 1 year |  |  |  |  |  |
stock repurchase 2023 plan | Treasury Stock, Common |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Treasury stock, shares, acquired | shares |  |  |  |  | 400,868 |  |  |
Treasury Stock, Value, Acquired, Cost Method |  |  |  |  | $ (39,000,000) |  |  |
Share awards vesting | Treasury Stock, Common |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Treasury stock, shares, acquired | shares |  |  |  |  | 35,897 | 31,351 | 31,317 |
Treasury Stock, Value, Acquired, Cost Method |  |  |  |  | $ (3,500,000) | $ (4,400,000) | $ (3,900,000) |

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