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Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands | Sep. 30, 2020 | Dec. 31, 2019
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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 | $ 3,003,029 | $ 2,857,250
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 12.71% | 12.36%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,654,073 | $ 1,617,886
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 | $ 1,535,925 | $ 1,502,323
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 | $ 3,003,029 | $ 3,001,736
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 12.71% | 12.99%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 2,008,517 | $ 1,964,576
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 | $ 1,890,369 | $ 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 | $ 3,471,898 | $ 3,367,403
Total Capital to Risk-Weighted Assets, Actual, Ratio | 14.69% | 14.57%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 2,481,109 | $ 2,426,829
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 | $ 2,362,961 | $ 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 | $ 3,003,029 | $ 3,001,736
Leverage Ratio, Actual, Ratio | 7.85% | 9.28%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,530,584 | $ 1,293,188
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 | $ 1,913,230 | $ 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 1 to Risk-Weighted Assets, Actual, Capital Amount | $ 3,010,290 | $ 2,958,326
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 12.76% | 12.82%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,651,191 | $ 1,615,206
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 | $ 1,533,249 | $ 1,499,834
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 | $ 3,010,290 | $ 2,958,326
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 12.76% | 12.82%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 2,005,018 | $ 1,961,322
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 | $ 1,887,076 | $ 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 | $ 3,246,159 | $ 3,090,993
Total Capital to Risk-Weighted Assets, Actual, Ratio | 13.76% | 13.40%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 2,476,787 | $ 2,422,809
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 | $ 2,358,845 | $ 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 | $ 3,010,290 | $ 2,958,326
Leverage Ratio, Actual, Ratio | 7.87% | 9.15%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,530,076 | $ 1,292,743
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 | $ 1,912,596 | $ 1,615,929
Leverage Ratio, Required to be Considered Well Capitalized, Ratio | 5.00% | 5.00%

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