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Capital and Regulatory Matters (Narrative) (Detail) |  |  |  |  | 1 Months Ended | 12 Months Ended |
--- | --- | --- | --- | --- | --- | --- | ---
Jul. 24, 2019 USD ($) | Mar. 17, 2017 USD ($) | Jan. 01, 2016 | Feb. 15, 2013 USD ($) $ / shares shares | Feb. 28, 2007 USD ($) | Dec. 31, 2019 USD ($) quarter $ / shares shares | Dec. 31, 2018 USD ($) $ / shares shares | Dec. 31, 2017 USD ($) shares | Jan. 01, 2019
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Subordinated notes |  |  |  |  |  | $ 98,865,000 | $ 98,708,000 |  |
Preferred stock, shares issued | shares |  |  |  |  |  | 6,000,000 | 6,000,000 |  |
Preferred stock, par value | $ / shares |  |  |  |  |  | $ 0.01 | $ 0.01 |  |
Proceeds from issuance of preferred stock |  |  |  | $ 144,500,000 |  |  |  |  |
Share repurchase authorized amount | $ 100,000,000.0 |  |  |  |  |  |  |  |
Stock repurchase program, period in force | 1 year |  |  |  |  |  |  |  |
Treasury stock, shares, acquired | shares |  |  |  |  |  | 716,062 | 1,037,982 | 1,149,555 |
Treasury Stock, Value, Acquired, Cost Method |  |  |  |  |  | $ (68,793,000) | $ (101,010,000) | $ (101,473,000) |
Aggregate dividends available for payment without prior regulatory approval |  |  |  |  |  | $ 682,900,000 |  |  |
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 A Preferred Stock [Member] |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Non-cumulative perpetual preferred stock, dividend rate percentage |  |  |  | 5.375% |  |  |  |  |
Preferred stock, shares issued | shares |  |  |  | 6,000,000 |  |  |  |  |
Series A Preferred Stock, Liquidation preference value |  |  |  | $ 150,000,000.0 |  | $ 25 | 25 |  |
Preferred stock, par value | $ / shares |  |  |  | $ 0.01 |  |  |  |  |
Liquidation preference per share | $ / shares |  |  |  | $ 25 |  |  |  |  |
Subordinated Debt [Member] |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Subordinated notes |  | $ 100,000,000 |  |  | $ 100,000,000 |  |  |  |
Subordinated notes interest rate |  | 4.50% |  |  | 5.75% |  |  |  |
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] |  |  |  |  |  |  |  |  |
Non-cumulative perpetual preferred stock |  |  |  |  |  | $ 144,500,000 | $ 144,500,000 |  |
Non-cumulative perpetual preferred stock, dividend rate percentage |  |  |  |  |  | 5.375% | 5.375% |  |
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio |  |  |  |  |  |  | 6.375% |  | 4.50%
Capital Conservation Buffer |  |  |  |  |  |  |  |  | 2.50%
Effective common equity tier one capital to risk weighted assets, ratio upon full implementation |  |  |  |  |  |  |  |  | 7.00%
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets |  |  |  |  |  |  |  |  | 6.00%
Effective tier one capital ratio, upon full implementation |  |  |  |  |  |  |  |  | 8.50%
Minimum ratio of Total capital to risk weighted assets |  |  |  |  |  | 14.57% | 14.64% |  | 8.00%
Minimum leverage ratio |  |  |  |  |  | 9.28% | 9.06% |  |
Tier one capital conservation buffer to risk weighted assets required under regulatory |  |  | 0.625% |  |  |  |  |  |
Tier one risk based capital conservation buffer initial requirements phasing duration |  |  | 4 years |  |  |  |  |  |
Cullen/Frost [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Tier one risk based capital to risk weighted assets, ratio upon full implementation |  |  |  |  |  |  |  |  | 10.50%
Minimum leverage ratio |  |  |  |  |  |  |  |  | 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 |  |  |  |  |  | $ 133,000,000.0 | $ 133,000,000 |  |
Cullen/Frost [Member] | Subordinated Debt [Member] |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Permissible portion of subordinated note included in total capital |  |  |  |  |  | 100,000,000.0 | 100,000,000 |  |
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 |  |  |  |  |  |  | 6.375% |  | 4.50%
Capital Conservation Buffer |  |  |  |  |  |  |  |  | 2.50%
Effective common equity tier one capital to risk weighted assets, ratio upon full implementation |  |  |  |  |  |  |  |  | 7.00%
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets |  |  |  |  |  |  |  |  | 6.00%
Effective tier one capital ratio, upon full implementation |  |  |  |  |  |  |  |  | 8.50%
Minimum ratio of Total capital to risk weighted assets |  |  |  |  |  | 13.40% | 13.40% |  | 8.00%
Minimum leverage ratio |  |  |  |  |  | 9.15% | 8.93% |  |
Tier one capital conservation buffer to risk weighted assets required under regulatory |  |  | 0.625% |  |  |  |  |  |
Tier one risk based capital conservation buffer initial requirements phasing duration |  |  | 4 years |  |  |  |  |  |
Frost Bank [Member] | Minimum [Member] |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Tier one risk based capital to risk weighted assets, ratio upon full implementation |  |  |  |  |  |  |  |  | 10.50%
Minimum leverage ratio |  |  |  |  |  |  |  |  | 4.00%
stock repurchase 2019 plan [Member] |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Treasury Stock, Value, Acquired, Cost Method |  |  |  |  |  | $ (17,200,000) |  |  |
stock repurchase 2019 plan [Member] | Treasury Stock [Member] |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Treasury stock, shares, acquired | shares |  |  |  |  |  | 202,724 |  |  |
stock repurchase 2017 plan [Member] |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Treasury Stock, Value, Acquired, Cost Method |  |  |  |  |  | $ (50,000,000.0) | $ (100,000,000.0) |  |
stock repurchase 2017 plan [Member] | Treasury Stock [Member] |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Treasury stock, shares, acquired | shares |  |  |  |  |  | 496,307 | 1,027,292 |  |
stock repurchase 2016 plan [Member] |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Treasury Stock, Value, Acquired, Cost Method |  |  |  |  |  |  |  | $ (100,000,000.0) |
stock repurchase 2016 plan [Member] | Treasury Stock [Member] |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Treasury stock, shares, acquired | shares |  |  |  |  |  |  |  | 1,134,966 |

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