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Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands | Sep. 30, 2024 | Dec. 31, 2023
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Cullen/Frost [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount | $ 4,241,732 | $ 4,036,945
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 13.55% | 13.25%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,190,654 | $ 2,132,516
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 7.00% | 7.00%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 4,387,184 | $ 4,182,397
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 14.02% | 13.73%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,660,080 | $ 2,589,484
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 8.50% | 8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 1,877,704 | $ 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 | $ 4,850,001 | $ 4,625,760
Total Capital to Risk-Weighted Assets, Actual, Ratio | 15.50% | 15.18%
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 3,285,982 | $ 3,198,774
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 10.50% | 10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 3,129,506 | $ 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 | $ 4,387,184 | $ 4,182,397
Leverage Ratio, Actual, Ratio | 8.80% | 8.35%
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 1,995,300 | $ 2,003,020
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 4.00% | 4.00%
Frost Bank [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount | $ 4,303,872 | $ 4,057,111
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 13.76% | 13.33%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,189,228 | $ 2,129,784
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 7.00% | 7.00%
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Capital Amount | $ 2,032,855 | $ 1,977,656
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Ratio | 6.50% | 6.50%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 4,303,872 | $ 4,057,111
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 13.76% | 13.33%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,658,349 | $ 2,586,166
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 8.50% | 8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 2,501,975 | $ 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 | $ 4,606,689 | $ 4,320,474
Total Capital to Risk-Weighted Assets, Actual, Ratio | 14.73% | 14.20%
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 3,283,842 | $ 3,194,676
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 10.50% | 10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 3,127,469 | $ 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 | $ 4,303,872 | $ 4,057,111
Leverage Ratio, Actual, Ratio | 8.63% | 8.10%
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 1,995,630 | $ 2,003,152
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 4.00% | 4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount | $ 2,494,538 | $ 2,503,940
Leverage Ratio, Required to be Considered Well Capitalized, Ratio | 5.00% | 5.00%

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