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Capital and Regulatory Matters (Narrative) (Detail) |  |  |  |  |  | 3 Months Ended | 12 Months Ended |  |  |
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Jan. 26, 2022 USD ($) | Nov. 19, 2020 USD ($) $ / shares shares | Mar. 16, 2020 USD ($) $ / shares | Mar. 17, 2017 USD ($) | Feb. 15, 2013 USD ($) $ / shares shares | Dec. 31, 2021 USD ($) $ / shares Rate shares | Mar. 31, 2020 USD ($) shares | Dec. 31, 2021 USD ($) quarter $ / shares Rate shares | Dec. 31, 2020 USD ($) $ / shares Rate shares | Dec. 31, 2019 USD ($) shares | Dec. 31, 2004 USD ($) | Jan. 01, 2020 USD ($) | Jan. 01, 2019 USD ($) Rate | Dec. 31, 2018 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit loss optional transitional period under CECL |  |  |  |  |  |  |  | 5 years |  |  |  |  |  |
Payments for Repurchase of Trust Preferred Securities |  |  |  |  |  |  |  | $ 13,403,000 |  |  |  |  |  |
Subordinated notes |  |  |  |  |  | $ 99,178,000 |  | $ 99,178,000 | $ 99,021,000 |  |  |  |  |
Cumulative effect of accounting change |  |  |  |  |  |  |  |  |  | $ (29,252,000) |  |  |  | $ (14,672,000)
Credit loss transitional period in years under CECL |  |  |  |  |  |  |  | 3 years |  |  |  |  |  |
CECL Transition Amount |  |  |  |  |  | 61,600,000 |  | $ 61,600,000 |  |  |  |  |  |
Estimated Incremental Effect of CECL |  |  |  |  |  |  |  | $ 32,400,000 |  |  |  |  |  |
Stock Redeemed or Called During Period, Value |  |  |  |  |  |  |  |  | $ 150,000,000 |  |  |  |  |
Treasury stock, shares, acquired | shares |  |  |  |  |  |  |  | 31,317 | 206,951 | 716,062 |  |  |  |
Treasury Stock, Value, Acquired, Cost Method |  |  |  |  |  |  |  | $ (3,864,000) | $ (15,785,000) | $ (68,793,000) |  |  |  |
Aggregate dividends available for payment without prior regulatory approval |  |  |  |  |  | $ 494,100,000 |  | $ 494,100,000 |  |  |  |  |  |
Maximum number of quarterly periods by which the corporation has the right to defer interest payments on junior subordinated deferrable interest debentures | quarter |  |  |  |  |  |  |  | 20 |  |  |  |  |  |
Subsequent Event |  |  |  |  |  |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Share repurchase authorized amount | $ 100,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Stock repurchase program, period in force | 1 year |  |  |  |  |  |  |  |  |  |  |  |  |
WNB Capital Trust I [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Trust preferred securities |  |  |  |  |  |  |  |  |  |  | $ 13,000,000 |  |  |
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)
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 |  |  |  |  |  |  |  |  |
Preferred Stock, Aggregate Liquidation Preference, Value |  |  |  |  | $ 150,000,000 |  |  |  |  |  |  |  |  |
Preferred Stock, Par or Stated Value Per Share | $ / shares |  |  |  |  | $ 0.01 |  |  |  |  |  |  |  |  |
Liquidation preference per share | $ / shares |  |  | $ 25 |  | $ 25 |  |  |  |  |  |  |  |  |
Payments of Stock Issuance Costs |  |  |  |  | $ 5,500,000 |  |  |  |  |  |  |  |  |
Proceeds from issuance of preferred stock |  |  |  |  | $ 144,500,000 |  |  |  |  |  |  |  |  |
Stock Redeemed or Called During Period, Value |  |  | $ 150,000,000 |  |  |  |  |  |  |  |  |  |  |
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 |  | 150,000 | 150,000 |  |  |  |  |
Preferred Stock, Aggregate Liquidation Preference, Value |  | $ 150,000,000 |  |  |  | $ 1,000 |  | $ 1,000 | $ 1,000 |  |  |  |  |
Preferred Stock, Par or Stated Value Per Share | $ / shares |  | $ 0.01 |  |  |  | $ 0.01 |  | $ 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] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional Tier 1 Capital beyond Common Equity Tier 1 |  |  |  |  |  | $ 145,500,000 |  | $ 145,500,000 | $ 145,500,000 |  |  |  |  |
Non-cumulative perpetual preferred stock, dividend rate percentage |  | 4.45% |  |  |  |  |  |  |  |  |  |  |  |
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% |  |  |  | 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% |  |  |  | 8.50% |
Minimum ratio of Total capital to risk weighted assets | Rate |  |  |  |  |  | 15.45% |  | 15.45% | 15.44% |  |  |  | 8.00% |
Tier one risk based capital to risk weighted assets, ratio upon full implementation |  |  |  |  |  | 10.50% |  | 10.50% | 10.50% |  |  |  | 10.50% |
Minimum leverage ratio |  |  |  |  |  | 4.00% |  | 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 |  |  |  |  |  |  |  | $ 120,000,000 | $ 133,000,000 |  |  |  |  |
Cullen/Frost [Member] | Trust Preferred Securities Tier Two Capital Allowable Portion [Member] | WNB Capital Trust I [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Payments for Repurchase of Trust Preferred Securities |  |  |  |  |  | $ 13,000,000 |  |  |  |  |  |  |  |
Cullen/Frost [Member] | Subordinated Debt [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Permissible Portion Of Subordinated Note Included In Capital |  |  |  |  |  | 100,000,000 |  | 100,000,000 | 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 | $ 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% |  |  |  | 8.50% |
Minimum ratio of Total capital to risk weighted assets | Rate |  |  |  |  |  | 13.61% |  | 13.61% | 13.76% |  |  |  | 8.00% |
Tier one risk based capital to risk weighted assets, ratio upon full implementation |  |  |  |  |  | 10.50% |  | 10.50% | 10.50% |  |  |  | 10.50% |
Minimum leverage ratio |  |  |  |  |  | 4.00% |  | 4.00% | 4.00% |  |  |  | 4.00% |
Prior stock repurchase plans [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Treasury Stock, Value, Acquired, Cost Method |  |  |  |  |  |  | $ (13,700,000) |  |  | $ (67,200,000) |  |  |  |
Prior stock repurchase plans [Member] | Treasury Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Treasury stock, shares, acquired | shares |  |  |  |  |  |  | 177,834 |  |  | 699,031 |  |  |  |
stock repurchase 2021 plan | Treasury Stock [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |
Treasury stock, shares, acquired | shares |  |  |  |  |  |  |  | 0 |  |  |  |  |  |

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- Definition The per share liquidation preference (or restrictions) of nonredeemable preferred stock (or preferred stock redeemable solely at the option of the issuer) that has a preference in involuntary liquidation considerably in excess of the par or stated value of the shares. The liquidation preference is the difference between the preference in liquidation and the par or stated values of the share. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 235 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.4-08(d)) -URI http://asc.fasb.org/extlink&oid=120395691&loc=d3e23780-122690 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 3 -URI http://asc.fasb.org/extlink&oid=123467817&loc=d3e21475-112644 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(28)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 13 -Subparagraph (h) -URI http://asc.fasb.org/extlink&oid=123467817&loc=SL123496158-112644 Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 4 -URI http://asc.fasb.org/extlink&oid=123467817&loc=d3e21484-112644 + Details
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- Definition Total number of nonredeemable preferred shares (or preferred stock redeemable solely at the option of the issuer) issued to shareholders (includes related preferred shares that were issued, repurchased, and remain in the treasury). May be all or portion of the number of preferred shares authorized. Excludes preferred shares that are classified as debt. + 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(28)) -URI http://asc.fasb.org/extlink&oid=120391452&loc=d3e13212-122682 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 505 -SubTopic 10 -Section 50 -Paragraph 13 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=123467817&loc=SL123496158-112644 + Details
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