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Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($)
$ in Thousands
Sep. 30, 2021
Dec. 31, 2020
Cullen/Frost [Member]    
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]    
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount $ 3,300,979 $ 3,058,447
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio 13.42% 12.86%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount $ 1,722,459 $ 1,664,867
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 $ 1,599,426 $ 1,545,948
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 $ 3,446,431 $ 3,203,899
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio 14.01% 13.47%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount $ 2,091,558 $ 2,021,624
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 $ 1,968,525 $ 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 $ 3,911,272 $ 3,672,912
Total Capital to Risk-Weighted Assets, Actual, Ratio 15.90% 15.44%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount $ 2,583,689 $ 2,497,300
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 $ 2,460,656 $ 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 $ 3,446,431 $ 3,203,899
Leverage Ratio, Actual, Ratio 7.52% 8.07%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount $ 1,833,156 $ 1,589,004
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 $ 2,291,445 $ 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 1 to Risk-Weighted Assets, Actual, Capital Amount $ 3,215,116 $ 3,030,093
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio 13.09% 12.77%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount $ 1,719,451 $ 1,661,620
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 $ 1,596,633 $ 1,542,933
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 $ 3,215,116 $ 3,030,093
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio 13.09% 12.77%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount $ 2,087,905 $ 2,017,682
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 $ 1,965,087 $ 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 $ 3,446,957 $ 3,266,106
Total Capital to Risk-Weighted Assets, Actual, Ratio 14.03% 13.76%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount $ 2,579,177 $ 2,492,430
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 $ 2,456,359 $ 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 $ 3,215,116 $ 3,030,093
Leverage Ratio, Actual, Ratio 7.02% 7.63%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount $ 1,832,498 $ 1,588,200
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 $ 2,290,623 $ 1,985,250
Leverage Ratio, Required to be Considered Well Capitalized, Ratio 5.00% 5.00%