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Regulatory Matters (Actual and Required Capital Ratios) (Detail) - USD ($)
$ in Thousands
Dec. 31, 2020
Dec. 31, 2019
Jan. 01, 2019
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%