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Capital and Regulatory Matters (Narrative) (Detail) |  |  |  |  |  | 12 Months Ended |  |  |  |
--- | --- | --- | --- | --- | --- | --- | --- | --- | --- | ---
Nov. 19, 2020 USD ($) $ / shares shares | Mar. 16, 2020 USD ($) $ / shares | Jul. 24, 2019 USD ($) | Mar. 17, 2017 USD ($) | Feb. 15, 2013 USD ($) $ / shares shares | Dec. 31, 2020 USD ($) quarter $ / shares Rate shares | Dec. 31, 2019 USD ($) $ / shares Rate shares | Dec. 31, 2018 USD ($) shares | Jan. 01, 2020 USD ($) | Jan. 01, 2019 USD ($) Rate | Jan. 01, 2018 USD ($) | Dec. 31, 2017 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Credit loss transitional period in years under CECL |  |  |  |  |  | 5 years |  |  |  |  |  |
Subordinated notes |  |  |  |  |  | $ 99,021,000 | $ 98,865,000 |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  |  | $ (29,252,000) | $ (14,672,000) |  |  |  | $ (2,285,000)
CECL Transition Amount |  |  |  |  |  | 63,700,000 |  |  |  |  |  |
Estimated Incremental Effect of CECL |  |  |  |  |  | 34,400,000 |  |  |  |  |  |
Stock Redeemed or Called During Period, Value |  |  |  |  |  | $ 150,000,000 |  |  |  |  |  |
Share repurchase authorized amount |  |  | $ 100,000,000.0 |  |  |  |  |  |  |  |  |
Stock repurchase program, period in force |  |  | 1 year |  |  |  |  |  |  |  |  |
Treasury stock, shares, acquired | shares |  |  |  |  |  | 206,951 | 716,062 | 1,037,982 |  |  |  |
Treasury Stock, Value, Acquired, Cost Method |  |  |  |  |  | $ (15,785,000) | $ (68,793,000) | $ (101,010,000) |  |  |  |
Aggregate dividends available for payment without prior regulatory approval |  |  |  |  |  | $ 503,900,000 |  |  |  |  |  |
Maximum number of quarterly periods by which the corporation has the right to defer interest payments on junior subordinated deferrable interest debentures | quarter |  |  |  |  |  | 20 |  |  |  |  |  |
Preferred Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Stock Redeemed or Called During Period, Value |  |  |  |  |  | $ 144,486,000 |  |  |  |  |  |
Retained Earnings [Member] |  |  |  |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  |  | $ (29,252,000) | (14,672,000) | $ (29,300,000) | $ (14,700,000) | $ (2,300,000) | $ (2,285,000)
Stock Redeemed or Called During Period, Value |  |  |  |  |  | $ 5,514,000 |  |  |  |  |  |
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 |  | 6,000,000 |  |  |  |  |
Preferred Stock, Aggregate Liquidation Preference, Value |  |  |  |  | $ 150,000,000.0 |  | $ 25 |  |  |  |  |
Preferred Stock, Par or Stated Value Per Share | $ / shares |  |  |  |  | $ 0.01 |  | $ 0.01 |  |  |  |  |
Liquidation preference per share | $ / shares |  | $ 25 |  |  | $ 25 |  |  |  |  |  |  |
Payments of Stock Issuance Costs |  |  |  |  | $ 5,500,000 |  |  |  |  |  |  |
Stock Redeemed or Called During Period, Value |  | $ 150,000,000.0 |  |  |  |  |  |  |  |  |  |
Proceeds from issuance of preferred stock |  |  |  |  | $ 144,500,000 |  |  |  |  |  |  |
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 |  |  |  |  |  |
Preferred Stock, Aggregate Liquidation Preference, Value | $ 150,000,000.0 |  |  |  |  | $ 1,000 |  |  |  |  |  |
Preferred Stock, Par or Stated Value Per Share | $ / shares | $ 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] |  |  |  |  |  |  |  |  |  |  |  |
Non-cumulative perpetual preferred stock |  |  |  |  |  | $ 145,500,000 | $ 144,500,000 |  |  |  |  |
Non-cumulative perpetual preferred stock, dividend rate percentage | 4.45% |  |  |  |  |  | 5.375% |  |  |  |  |
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 |  |  |  |  |  | 15.44% | 14.57% |  |  | 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 |  |  |  |  |  | $ 133,000,000.0 | $ 133,000,000.0 |  |  |  |  |
Cullen/Frost [Member] | Subordinated Debt [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 |  |  |  |  |
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 |  |  |  |  |  | 13.76% | 13.40% |  |  | 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 2019 plan [Member] |  |  |  |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |
Treasury Stock, Value, Acquired, Cost Method |  |  |  |  |  | $ (13,700,000) | $ (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 |  |  |  |  |  | 177,834 | 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 |  |  |  |

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- Definition Including the current and noncurrent portions, carrying value as of the balance sheet date of subordinated debt (with initial maturities beyond one year or beyond the operating cycle if longer). Subordinated debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(19)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(16)) -URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 944 -SubTopic 210 -Section S99 -Paragraph 1 -Subparagraph (SX 210.7-03(a)(16)) -URI http://asc.fasb.org/extlink&oid=120400017&loc=d3e572229-122910 Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(22)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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- Definition Ratio of minimum Tier 1 risk-based capital to risk-weighted assets required for capital adequacy as defined by regulatory framework. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1B -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958570-112826 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -Subparagraph (c)(1) -URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826 + Details
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- Definition Number of shares that have been repurchased during the period and are being held in treasury. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02(29)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 + Details
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- Definition Equity impact of the cost of common and preferred stock that were repurchased during the period. Recorded using the cost method. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 2 -URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 30 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6405813&loc=d3e23239-112655 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.3-04) -URI http://asc.fasb.org/extlink&oid=120397183&loc=d3e187085-122770 + Details
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