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Regulatory Matters (Actual and Required Capital Ratios) (Detail) - USD ($) $ in Thousands | Dec. 31, 2021 | Dec. 31, 2020 | 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 | $ 3,371,043 | $ 3,058,447 |
Common Equity Tier One Risk-Based Capital Ratio, Actual | 13.13% | 12.86% |
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 1,796,549 | $ 1,664,867 |
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 | $ 1,668,224 | $ 1,545,948 |
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 | $ 3,516,495 | $ 3,203,899 |
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 13.70% | 13.47% |
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 2,181,523 | $ 2,021,624 |
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 | $ 2,053,198 | $ 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 | $ 3,966,244 | $ 3,672,912 |
Total Capital to Risk-Weighted Assets, Actual, Ratio | 15.45% | 15.44% | 8.00%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 2,694,823 | $ 2,497,300 |
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 | $ 2,566,498 | $ 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 | $ 3,516,495 | $ 3,203,899 |
Leverage Ratio, Actual, Ratio | 7.34% | 8.07% |
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 1,917,533 | $ 1,589,004 |
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 | $ 2,396,917 | $ 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 One to Risk-Weighted Assets, Actual, Capital Amount | $ 3,261,532 | $ 3,030,093 |
Common Equity Tier One Risk-Based Capital Ratio, Actual | 12.72% | 12.77% |
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 1,795,221 | $ 1,661,620 |
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio | 7.00% | 7.00% |
Common Equity Tier One Capital Required To Be Well Capitalized, Capital Amount | $ 1,666,991 | $ 1,542,933 |
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 | $ 3,261,532 | $ 3,030,093 |
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 12.72% | 12.77% |
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 2,179,911 | $ 2,017,682 |
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 | $ 2,051,681 | $ 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 | $ 3,491,281 | $ 3,266,106 |
Total Capital to Risk-Weighted Assets, Actual, Ratio | 13.61% | 13.76% | 8.00%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 2,692,831 | $ 2,492,430 |
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 | $ 2,564,601 | $ 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 | $ 3,261,532 | $ 3,030,093 |
Leverage Ratio, Actual, Ratio | 6.80% | 7.63% |
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount | $ 1,917,679 | $ 1,588,200 |
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 | $ 2,397,099 | $ 1,985,250 |
Leverage Ratio, Required to be Considered Well Capitalized, Ratio | 5.00% | 5.00% |

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