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Capital and Regulatory Matters - Additional Information (Detail) $ in Thousands |  |  | 3 Months Ended | 9 Months Ended | 12 Months Ended
--- | --- | --- | --- | --- | ---
Jul. 24, 2019 USD ($) | Oct. 24, 2017 USD ($) | Sep. 30, 2019 USD ($) shares | Jun. 30, 2019 USD ($) shares | Dec. 31, 2018 USD ($) shares | Sep. 30, 2018 shares | Sep. 30, 2019 USD ($) quarter shares | Sep. 30, 2018 USD ($) shares | Dec. 31, 2018 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Stock Repurchase Program, Authorized Amount | $ 100,000 | $ 150,000 |  |  |  |  |  |  |
Stock Repurchase Program, Period in Force | 1 year | 2 years |  |  |  |  |  |  |
Treasury stock, shares, acquired | shares |  |  | 202,724 |  |  | 0 | 704,382 | 601 |
Treasury Stock, Value, Acquired, Cost Method |  |  | $ (17,190) |  |  |  | $ (67,709) | $ (70) |
Qualified subordinated debt |  |  | 98,826 |  | $ 98,708 |  | 98,826 |  | $ 98,708
Maximum dividends available without prior regulatory approval |  |  | 633,800 |  |  |  | $ 633,800 |  |
Maximum number of quarterly periods by which the corporation has the right to defer interest payments on junior subordinated deferrable interest debentures | quarter |  |  |  |  |  |  | 20 |  |
Trust Preferred Securities Tier One Excluded Portion [Member] |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Trust preferred securities |  |  |  |  |  |  | $ 133,000 |  | 133,000
Cullen/Frost [Member] |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Preferred stock value issued include for regulatory tier one capital calculations |  |  | 144,500 |  | 144,500 |  | $ 144,500 |  | $ 144,500
Preferred stock dividend rate percentage |  |  |  |  |  |  | 5.375% |  | 5.375%
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 |  | $ 0
Stock Repurchase 2017 Plan | Treasury Stock [Member] |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Treasury stock, shares, acquired | shares |  |  |  | 496,307 | 1,027,292 |  |  |  |
Treasury Stock, Value, Acquired, Cost Method |  |  |  | $ (50,000) | $ (100,000) |  |  |  |
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 |  |  |  |  |  |
Treasury Stock, Value, Acquired, Cost Method |  |  | $ (17,200) |  |  |  |  |  |
Subordinated Debt [Member] |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |
Qualified subordinated debt |  |  | $ 100,000 |  | $ 100,000 |  | $ 100,000 |  | $ 100,000

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