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Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands | Jun. 30, 2021 | Dec. 31, 2020
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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 | $ 3,239,651 | $ 3,058,447
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 13.60% | 12.86%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,667,710 | $ 1,664,867
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased In, Ratio | 7.00% | 7.00%
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Capital Amount | $ 1,548,588 | $ 1,545,948
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 | $ 3,385,103 | $ 3,203,899
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 14.21% | 13.47%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 2,025,077 | $ 2,021,624
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio | 8.50% | 8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 1,905,954 | $ 1,902,705
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 | $ 3,851,531 | $ 3,672,912
Total Capital to Risk-Weighted Assets, Actual, Ratio | 16.17% | 15.44%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 2,501,565 | $ 2,497,300
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio | 10.50% | 10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 2,382,443 | $ 2,378,381
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00%
Leverage Ratio, Actual, Capital Amount | $ 3,385,103 | $ 3,203,899
Leverage Ratio, Actual, Ratio | 7.60% | 8.07%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,781,253 | $ 1,589,004
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Ratio | 4.00% | 4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount | $ 2,226,566 | $ 1,986,255
Leverage Ratio, Required to be Considered Well Capitalized, Ratio | 5.00% | 5.00%
Frost Bank [Member] |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount | $ 3,166,299 | $ 3,030,093
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 13.32% | 12.77%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,664,468 | $ 1,661,620
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased In, Ratio | 7.00% | 7.00%
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Capital Amount | $ 1,545,578 | $ 1,542,933
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 | $ 3,166,299 | $ 3,030,093
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 13.32% | 12.77%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 2,021,140 | $ 2,017,682
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio | 8.50% | 8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 1,902,249 | $ 1,898,995
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 | $ 3,399,727 | $ 3,266,106
Total Capital to Risk-Weighted Assets, Actual, Ratio | 14.30% | 13.76%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 2,496,702 | $ 2,492,430
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio | 10.50% | 10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 2,377,812 | $ 2,373,743
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00%
Leverage Ratio, Actual, Capital Amount | $ 3,166,299 | $ 3,030,093
Leverage Ratio, Actual, Ratio | 7.11% | 7.63%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,780,416 | $ 1,588,200
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Ratio | 4.00% | 4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount | $ 2,225,521 | $ 1,985,250
Leverage Ratio, Required to be Considered Well Capitalized, Ratio | 5.00% | 5.00%

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