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Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands | Jun. 30, 2025 | Dec. 31, 2024
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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 | $ 4,522,416 | $ 4,343,666
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 13.98% | 13.62%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,264,455 | $ 2,232,822
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 | $ 4,667,868 | $ 4,489,118
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 14.43% | 14.07%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,749,695 | $ 2,711,283
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 | $ 1,940,961 | $ 1,913,847
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 | $ 5,135,590 | $ 4,954,136
Total Capital to Risk-Weighted Assets, Actual, Ratio | 15.88% | 15.53%
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 3,396,683 | $ 3,349,232
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 | $ 3,234,936 | $ 3,189,745
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00%
Leverage Ratio, Actual, Capital Amount | $ 4,667,868 | $ 4,489,118
Leverage Ratio, Actual, Ratio | 8.98% | 8.63%
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,080,263 | $ 2,079,715
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 | $ 4,536,485 | $ 4,387,862
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 14.03% | 13.76%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,263,613 | $ 2,231,710
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 | $ 2,101,926 | $ 2,072,302
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 | $ 4,536,485 | $ 4,387,862
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 14.03% | 13.76%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,748,673 | $ 2,709,934
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 | $ 2,586,986 | $ 2,550,526
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 | $ 4,864,207 | $ 4,692,880
Total Capital to Risk-Weighted Assets, Actual, Ratio | 15.04% | 14.72%
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 3,395,419 | $ 3,347,565
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 | $ 3,233,732 | $ 3,188,157
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00%
Leverage Ratio, Actual, Capital Amount | $ 4,536,485 | $ 4,387,862
Leverage Ratio, Actual, Ratio | 8.72% | 8.44%
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount | $ 2,080,532 | $ 2,079,965
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Ratio | 4.00% | 4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount | $ 2,600,665 | $ 2,599,956
Leverage Ratio, Required to be Considered Well Capitalized, Ratio | 5.00% | 5.00%

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