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v3.20.2

Capital and Regulatory Matters - Additional Information (Detail) |  |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Mar. 16, 2020 USD ($) shares | Jul. 24, 2019 USD ($) | Feb. 15, 2013 USD ($) | Sep. 30, 2020 USD ($) shares | Mar. 31, 2020 USD ($) shares | Sep. 30, 2019 USD ($) shares | Sep. 30, 2020 USD ($) quarter shares | Sep. 30, 2019 USD ($) shares | Dec. 31, 2019 USD ($) | Jan. 01, 2020 USD ($) | Jan. 01, 2019 USD ($) | Dec. 31, 2018 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  |  |  |  | $ (29,252,000) |  |  | $ (14,672,000)
CECL Transition Amount |  |  |  | $ (63,700,000) |  |  | $ (63,700,000) |  |  |  |  |
Estimated Incremental Effect of CECL |  |  |  |  |  |  | $ (34,400,000) |  |  |  |  |
Proceeds from Issuance of Preferred Stock and Preference Stock |  |  | $ 5,500,000 |  |  |  |  |  |  |  |  |
Stock Redeemed or Called During Period, 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 |  |  |  |  |
Stock Repurchase Program, Authorized Amount |  | $ 100,000,000.0 |  |  |  |  |  |  |  |  |  |
Stock Repurchase Program, Period in Force |  | 1 year |  |  |  |  |  |  |  |  |  |
Treasury Stock, Shares, Acquired | shares |  |  |  | 400 |  | 202,724 | 182,225 | 704,382 |  |  |  |
Treasury Stock, Value, Acquired, Cost Method |  |  |  | $ (27,000) |  | $ (17,190,000) | $ (14,000,000) | $ (67,709,000) |  |  |  |
Maximum dividends available without prior regulatory approval |  |  |  | 482,100,000 |  |  | $ 482,100,000 |  |  |  |  |
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 |  |  |  |  |  |  |  | 25 |  |  |
Cullen/Frost [Member] |  |  |  |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Additional Tier 1 Capital beyond Common Equity Tier 1 |  |  |  | 0 |  |  | $ 0 |  | 144,500,000 |  |  |
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 |  | 0 |  |  |
stock repurchase 2019 plan [Member] | Treasury Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Treasury Stock, Shares, Acquired | shares |  |  |  |  | 177,834 | 202,724 |  |  |  |  |  |
Treasury Stock, Value, Acquired, Cost Method |  |  |  |  | $ (13,700,000) | $ (17,200,000) |  |  |  |  |  |
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 |  | 100,000,000.0 |  |  |
Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Proceeds from Issuance of Preferred Stock and Preference Stock |  |  | $ 144,500,000 |  |  |  |  |  |  |  |  |
Preferred stock, redemption amount | 144,500,000 |  |  |  |  |  | 144,486,000 |  |  |  |  |
Retained Earnings [Member] |  |  |  |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  |  |  |  | $ (29,252,000) | $ (29,300,000) | $ (14,700,000) | $ (14,672,000)
Preferred stock, redemption amount | $ 5,500,000 |  |  |  |  |  | $ 5,514,000 |  |  |  |  |

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