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Regulatory Matters (Actual and Required Capital Ratios) (Detail) - USD ($) $ in Thousands | Dec. 31, 2024 | Dec. 31, 2023 | Jan. 01, 2019
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Cullen/Frost [Member] |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |
Common Equity Tier One to Risk-Weighted Assets, Actual, Capital Amount | $ 4,343,666 | $ 4,036,945 |
Common Equity Tier One Risk-Based Capital Ratio, Actual | 13.62% | 13.25% |
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,232,822 | $ 2,132,516 |
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 7.00% | 7.00% | 7.00%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 4,489,118 | $ 4,182,397 |
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 14.07% | 13.73% |
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,711,283 | $ 2,589,484 |
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 8.50% | 8.50% | 8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 1,913,847 | $ 1,827,871 |
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 6.00% | 6.00% |
Total Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 4,954,136 | $ 4,625,760 |
Total Capital to Risk-Weighted Assets, Actual, Ratio | 15.53% | 15.18% | 8.00%
Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 3,349,232 | $ 3,198,774 |
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 10.50% | 10.50% | 10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 3,189,745 | $ 3,046,452 |
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00% |
Leverage Ratio, Actual, Capital Amount | $ 4,489,118 | $ 4,182,397 |
Leverage Ratio, Actual, Ratio | 8.63% | 8.35% |
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,079,715 | $ 2,003,020 |
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 4.00% | 4.00% | 4.00%
Frost Bank [Member] |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |
Common Equity Tier One to Risk-Weighted Assets, Actual, Capital Amount | $ 4,387,862 | $ 4,057,111 |
Common Equity Tier One Risk-Based Capital Ratio, Actual | 13.76% | 13.33% |
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,231,710 | $ 2,129,784 |
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 7.00% | 7.00% | 7.00%
Common Equity Tier One Capital Required To Be Well Capitalized, Capital Amount | $ 2,072,302 | $ 1,977,656 |
Common Equity Tier One Risk-Based Capital Ratio, Well Capitalized, Minimum | 6.50% | 6.50% |
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 4,387,862 | $ 4,057,111 |
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 13.76% | 13.33% |
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,709,934 | $ 2,586,166 |
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 8.50% | 8.50% | 8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 2,550,526 | $ 2,434,038 |
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 8.00% | 8.00% |
Total Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 4,692,880 | $ 4,320,474 |
Total Capital to Risk-Weighted Assets, Actual, Ratio | 14.72% | 14.20% | 8.00%
Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 3,347,565 | $ 3,194,676 |
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 10.50% | 10.50% | 10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 3,188,157 | $ 3,042,548 |
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00% |
Leverage Ratio, Actual, Capital Amount | $ 4,387,862 | $ 4,057,111 |
Leverage Ratio, Actual, Ratio | 8.44% | 8.10% |
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,079,965 | $ 2,003,152 |
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 4.00% | 4.00% | 4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount | $ 2,599,956 | $ 2,503,940 |
Leverage Ratio, Required to be Considered Well Capitalized, Ratio | 5.00% | 5.00% |

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