Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands	Jun. 30, 2020	Dec. 31, 2019
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 2,940,493	$ 2,857,250
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	12.48%	12.36%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,649,766	$ 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	$ 1,531,926	$ 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	$ 2,940,493	$ 3,001,736
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.48%	12.99%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,003,287	$ 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	$ 1,885,447	$ 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	$ 3,401,768	$ 3,367,403
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.43%	14.57%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,474,649	$ 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	$ 2,356,809	$ 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	$ 2,940,493	$ 3,001,736
Leverage Ratio, Actual, Ratio	8.01%	9.28%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,469,180	$ 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	$ 1,836,475	$ 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	$ 2,977,672	$ 2,958,326
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	12.66%	12.82%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,646,300	$ 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	$ 1,528,707	$ 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	$ 2,977,672	$ 2,958,326
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.66%	12.82%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,999,078	$ 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	$ 1,881,485	$ 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	$ 3,205,947	$ 3,090,993
Total Capital to Risk-Weighted Assets, Actual, Ratio	13.63%	13.40%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,469,450	$ 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	$ 2,351,857	$ 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	$ 2,977,672	$ 2,958,326
Leverage Ratio, Actual, Ratio	8.11%	9.15%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,468,312	$ 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	$ 1,835,391	$ 1,615,929
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%