Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands	Jun. 30, 2026	Dec. 31, 2025
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 4,663,850	$ 4,601,579
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	13.95%	14.06%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,340,319	$ 2,291,102
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio	7.00%	7.00%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 4,809,302	$ 4,747,031
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	14.38%	14.50%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,841,816	$ 2,782,053
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio	8.50%	8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 2,005,988	$ 1,963,802
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	6.00%	6.00%
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 5,263,839	$ 5,220,324
Total Capital to Risk-Weighted Assets, Actual, Ratio	15.74%	15.95%
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 3,510,479	$ 3,436,653
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio	10.50%	10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 3,343,313	$ 3,273,003
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 4,809,302	$ 4,747,031
Leverage Ratio, Actual, Ratio	9.06%	8.80%
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,124,182	$ 2,157,988
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Ratio	4.00%	4.00%
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 4,820,325	$ 4,687,844
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	14.42%	14.33%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,339,820	$ 2,290,525
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio	7.00%	7.00%
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Capital Amount	$ 2,172,690	$ 2,126,916
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	$ 4,820,325	$ 4,687,844
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	14.42%	14.33%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,841,210	$ 2,781,351
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio	8.50%	8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 2,674,080	$ 2,617,742
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	$ 5,154,862	$ 5,021,137
Total Capital to Risk-Weighted Assets, Actual, Ratio	15.42%	15.34%
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 3,509,730	$ 3,435,787
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Ratio	10.50%	10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 3,342,600	$ 3,272,178
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 4,820,325	$ 4,687,844
Leverage Ratio, Actual, Ratio	9.08%	8.69%
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,124,252	$ 2,158,485
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 2,655,315	$ 2,698,106
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%