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Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands | Sep. 30, 2023 | Dec. 31, 2022
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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 | $ 3,977,860 | $ 3,751,200
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 13.32% | 12.85%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,090,045 | $ 2,042,876
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 | $ 4,123,312 | $ 3,896,652
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 13.81% | 13.35%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,537,911 | $ 2,480,635
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 | $ 1,791,467 | $ 1,751,036
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 | $ 4,561,577 | $ 4,330,982
Total Capital to Risk-Weighted Assets, Actual, Ratio | 15.28% | 14.84%
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 3,135,067 | $ 3,064,313
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 | $ 2,985,778 | $ 2,918,394
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00%
Leverage Ratio, Actual, Capital Amount | $ 4,123,312 | $ 3,896,652
Leverage Ratio, Actual, Ratio | 8.17% | 7.29%
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,018,632 | $ 2,136,680
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 | $ 4,013,794 | $ 3,789,056
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 13.46% | 13.00%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,087,478 | $ 2,040,388
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 | $ 1,938,372 | $ 1,894,646
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 | $ 4,013,794 | $ 3,789,056
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 13.46% | 13.00%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,534,795 | $ 2,477,614
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 | $ 2,385,689 | $ 2,331,872
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 | $ 4,272,059 | $ 4,023,386
Total Capital to Risk-Weighted Assets, Actual, Ratio | 14.33% | 13.80%
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 3,131,217 | $ 3,060,583
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 | $ 2,982,112 | $ 2,914,841
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00%
Leverage Ratio, Actual, Capital Amount | $ 4,013,794 | $ 3,789,056
Leverage Ratio, Actual, Ratio | 7.96% | 7.09%
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,018,241 | $ 2,136,316
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 4.00% | 4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount | $ 2,522,801 | $ 2,670,395
Leverage Ratio, Required to be Considered Well Capitalized, Ratio | 5.00% | 5.00%

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