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Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands | Sep. 30, 2019 | Dec. 31, 2018
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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 | $ 2,788,313 | $ 2,642,475
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 12.35% | 12.27%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount |  | $ 1,372,573
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio |  | 6.375%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,580,199 | $ 1,507,139
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 | $ 1,467,328 | $ 1,358,171
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 | $ 2,932,799 | $ 2,786,961
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 12.99% | 12.94%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount |  | $ 1,695,532
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio |  | 7.875%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,918,813 | $ 1,830,098
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 | $ 1,805,942 | $ 1,671,595
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 | $ 3,302,858 | $ 3,152,593
Total Capital to Risk-Weighted Assets, Actual, Ratio | 14.63% | 14.64%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount |  | $ 2,126,143
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio |  | 9.875%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 2,370,298 | $ 2,260,709
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 | $ 2,257,427 | $ 2,089,494
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00%
Leverage Ratio, Actual, Capital Amount | $ 2,932,798 | $ 2,786,961
Leverage Ratio, Actual, Ratio | 9.36% | 9.06%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount |  | $ 1,231,028
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Ratio |  | 4.00%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,252,781 | $ 1,231,028
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 | $ 1,565,976 | $ 1,538,785
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 | $ 2,910,446 | $ 2,743,973
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio | 12.92% | 12.78%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount |  | $ 1,368,701
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio |  | 6.375%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,576,393 | $ 1,502,887
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 | $ 1,463,794 | $ 1,354,222
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 | $ 2,910,446 | $ 2,743,973
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio | 12.92% | 12.78%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount |  | $ 1,690,748
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio |  | 7.875%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,914,192 | $ 1,824,934
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 | $ 1,801,593 | $ 1,666,735
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 | $ 3,047,505 | $ 2,876,605
Total Capital to Risk-Weighted Assets, Actual, Ratio | 13.53% | 13.40%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount |  | $ 2,120,144
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio |  | 9.875%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 2,364,590 | $ 2,254,331
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 | $ 2,251,991 | $ 2,083,419
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio | 10.00% | 10.00%
Leverage Ratio, Actual, Capital Amount | $ 2,910,446 | $ 2,743,973
Leverage Ratio, Actual, Ratio | 9.30% | 8.93%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount |  | $ 1,229,650
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Ratio |  | 4.00%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount | $ 1,251,713 | $ 1,229,650
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 | $ 1,564,642 | $ 1,537,062
Leverage Ratio, Required to be Considered Well Capitalized, Ratio | 5.00% | 5.00%

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- Definition The amount of excess Tier 1 Risk Based Capital (which consists of the types of financial capital considered the most reliable and liquid, primarily Shareholders' equity. Examples of Tier 1 capital are common stock or unit, preferred stock or unit that is irredeemable and noncumulative, and retained earnings) divided by risk-weighted assets which are assets that are weighted for credit risk according to a formula used by the Reserve Bank. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 505 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=84167132&loc=SL5958568-112826 + Details
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