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Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands | Mar. 31, 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 | $ 4,102,330 | $ 4,036,945
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 13.41% | 13.25%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,140,804 | $ 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 | $ 4,247,782 | $ 4,182,397
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 13.89% | 13.73%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,599,548 | $ 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 | $ 1,834,975 | $ 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 | $ 4,694,792 | $ 4,625,760
Total Capital to Risk-Weighted Assets, Actual, Ratio | 15.35% | 15.18%
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 3,211,206 | $ 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 | $ 3,058,292 | $ 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 | $ 4,247,782 | $ 4,182,397
Leverage Ratio, Actual, Ratio | 8.44% | 8.35%
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,012,280 | $ 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 | $ 4,146,611 | $ 4,057,111
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 13.57% | 13.33%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,138,279 | $ 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 | $ 1,985,545 | $ 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 | $ 4,146,611 | $ 4,057,111
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 13.57% | 13.33%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,596,482 | $ 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 | $ 2,443,748 | $ 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 | $ 4,433,621 | $ 4,320,474
Total Capital to Risk-Weighted Assets, Actual, Ratio | 14.51% | 14.20%
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 3,207,419 | $ 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 | $ 3,054,685 | $ 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 | $ 4,146,611 | $ 4,057,111
Leverage Ratio, Actual, Ratio | 8.24% | 8.10%
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,011,864 | $ 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 | $ 2,514,830 | $ 2,503,940
Leverage Ratio, Required to be Considered Well Capitalized, Ratio | 5.00% | 5.00%

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