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Regulatory Matters (Actual and Required Capital Ratios) (Detail) - USD ($) $ in Thousands | Dec. 31, 2020 | Dec. 31, 2019 | Jan. 01, 2019
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Cullen/Frost [Member] |  |  |
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items] |  |  |
Common Equity Tier One to Risk-Weighted Assets, Actual, Capital Amount | $ 3,058,447 | $ 2,857,250 |
Common Equity Tier One Risk-Based Capital Ratio, Actual | 12.86% | 12.36% |
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 1,664,867 | $ 1,617,886 |
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 7.00% | 7.00% | 7.00%
Common Equity Tier One Capital Required To Be Well Capitalized, Capital Amount | $ 1,545,948 | $ 1,502,323 |
Common Equity Tier One Risk-Based Capital Ratio, Well Capitalized, Minimum | 6.50% | 6.50% |
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 3,203,899 | $ 3,001,736 |
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 13.47% | 12.99% |
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 2,021,624 | $ 1,964,576 |
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 8.50% | 8.50% | 8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 1,902,705 | $ 1,849,013 |
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 | $ 3,672,912 | $ 3,367,403 |
Total Capital to Risk-Weighted Assets, Actual, Ratio | 15.44% | 14.57% | 8.00%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 2,497,300 | $ 2,426,829 |
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 10.50% | 10.50% | 10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 2,378,381 | $ 2,311,266 |
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00% |
Leverage Ratio, Actual, Capital Amount | $ 3,203,899 | $ 3,001,736 |
Leverage Ratio, Actual, Ratio | 8.07% | 9.28% |
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 1,589,004 | $ 1,293,188 |
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 4.00% | 4.00% | 4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount | $ 1,986,255 | $ 1,616,485 |
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 One to Risk-Weighted Assets, Actual, Capital Amount | $ 3,030,093 | $ 2,958,326 |
Common Equity Tier One Risk-Based Capital Ratio, Actual | 12.77% | 12.82% |
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 1,661,620 | $ 1,615,206 |
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 7.00% | 7.00% | 7.00%
Common Equity Tier One Capital Required To Be Well Capitalized, Capital Amount | $ 1,542,933 | $ 1,499,834 |
Common Equity Tier One Risk-Based Capital Ratio, Well Capitalized, Minimum | 6.50% | 6.50% |
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount | $ 3,030,093 | $ 2,958,326 |
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 12.77% | 12.82% |
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 2,017,682 | $ 1,961,322 |
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 8.50% | 8.50% | 8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 1,898,995 | $ 1,845,950 |
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 | $ 3,266,106 | $ 3,090,993 |
Total Capital to Risk-Weighted Assets, Actual, Ratio | 13.76% | 13.40% | 8.00%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 2,492,430 | $ 2,422,809 |
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 10.50% | 10.50% | 10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount | $ 2,373,743 | $ 2,307,438 |
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00% |
Leverage Ratio, Actual, Capital Amount | $ 3,030,093 | $ 2,958,326 |
Leverage Ratio, Actual, Ratio | 7.63% | 9.15% |
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 1,588,200 | $ 1,292,743 |
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 4.00% | 4.00% | 4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount | $ 1,985,250 | $ 1,615,929 |
Leverage Ratio, Required to be Considered Well Capitalized, Ratio | 5.00% | 5.00% |

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