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Capital and Regulatory Matters - Additional Information (Detail) |  |  |  |  | 3 Months Ended | 6 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | --- | --- | ---
Jan. 27, 2021 USD ($) | Nov. 19, 2020 USD ($) $ / shares shares | Mar. 16, 2020 USD ($) shares | Feb. 15, 2013 USD ($) | Mar. 31, 2020 USD ($) shares | Jun. 30, 2021 USD ($) quarter $ / shares shares | Jun. 30, 2020 USD ($) shares | Dec. 31, 2020 USD ($) $ / shares shares
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
CECL Transition Amount |  |  |  |  |  | $ 62,800,000 |  | $ 63,700,000
Redemption of series A preferred stock (6,000,000 shares) | shares |  |  | 6,000,000 |  |  |  | 6,000,000 |
Preferred stock dividend rate percentage |  |  | 5.375% |  |  |  |  |
Preferred stock, redemption amount |  |  | $ 150,000,000.0 |  |  |  | $ 150,000,000 |
Proceeds from Issuance of Preferred Stock and Preference Stock |  |  |  | $ 5,500,000 |  |  |  |
Purchase of treasury stock (11,625 shares during 2021 and 181,825 shares during 2020) | shares |  |  |  |  |  | 11,625 | 181,825 |
Treasury Stock, Value, Acquired, Cost Method |  |  |  |  |  | $ (1,288,000) | $ (13,973,000) |
Maximum dividends available without prior regulatory approval |  |  |  |  |  | $ 396,200,000 |  |
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] |  |  |  |  |  |  |  |
Preferred stock liquidation preference value |  |  | 25 |  |  |  |  |
Series B Preferred Stock [Member] |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Preferred stock dividend rate percentage |  | 4.45% |  |  |  |  |  |
Preferred stock liquidation preference value |  | $ 150,000,000.0 |  |  |  | $ 1,000 |  | $ 1,000
Preferred stock, shares issued | shares |  | 150,000 |  |  |  | 150,000 |  | 150,000
Preferred stock, par value | $ / shares |  | $ 0.01 |  |  |  | $ 0.01 |  | $ 0.01
Preferred Stock, 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 and Preference Stock |  | $ 145,500,000 |  |  |  |  |  |
Series B Preferred Stock Equivalent Depositary Shares Member |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Preferred Stock, Liquidation Preference Per Share | $ / shares |  | $ 25 |  |  |  |  |  |
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
Preferred stock dividend rate percentage |  |  |  |  |  | 4.45% |  | 4.45%
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 |  | $ 133,000,000.0
Frost Bank [Member] |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Additional Tier 1 Capital beyond Common Equity Tier 1 |  |  |  |  |  | $ 0 |  | 0
stock repurchase 2019 plan [Member] | Treasury Stock [Member] |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Purchase of treasury stock (11,625 shares during 2021 and 181,825 shares during 2020) | shares |  |  |  |  | 177,834 |  |  |
Treasury Stock, Value, Acquired, Cost Method |  |  |  |  | $ (13,700,000) |  |  |
stock repurchase 2021 plan |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Stock Repurchase Program, Authorized Amount | $ 100,000,000.0 |  |  |  |  |  |  |
Stock Repurchase Program, Period in Force | 1 year |  |  |  |  |  |  |
stock repurchase 2021 plan | Treasury Stock [Member] |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Purchase of treasury stock (11,625 shares during 2021 and 181,825 shares during 2020) | shares |  |  |  |  |  | 0 |  |
Subordinated Debt [Member] | Cullen/Frost [Member] |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Permissible portion of subordinated note included in capital |  |  |  |  |  | $ 100,000,000.0 |  | $ 100,000,000.0
Preferred Stock [Member] |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Preferred stock, redemption amount |  |  | 144,500,000 |  |  |  | 144,486,000 |
Proceeds from Issuance of Preferred Stock and Preference Stock |  |  |  | $ 144,500,000 |  |  |  |
Retained Earnings [Member] |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |
Preferred stock, redemption amount |  |  | $ 5,500,000 |  |  |  | $ 5,514,000 |

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