Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands	Sep. 30, 2024	Dec. 31, 2023
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 4,241,732	$ 4,036,945
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	13.55%	13.25%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,190,654	$ 2,132,516
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	$ 4,387,184	$ 4,182,397
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	14.02%	13.73%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,660,080	$ 2,589,484
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	$ 1,877,704	$ 1,827,871
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	$ 4,850,001	$ 4,625,760
Total Capital to Risk-Weighted Assets, Actual, Ratio	15.50%	15.18%
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 3,285,982	$ 3,198,774
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	$ 3,129,506	$ 3,046,452
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 4,387,184	$ 4,182,397
Leverage Ratio, Actual, Ratio	8.80%	8.35%
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 1,995,300	$ 2,003,020
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	$ 4,303,872	$ 4,057,111
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	13.76%	13.33%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,189,228	$ 2,129,784
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	$ 2,032,855	$ 1,977,656
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	$ 4,303,872	$ 4,057,111
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.76%	13.33%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,658,349	$ 2,586,166
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	$ 2,501,975	$ 2,434,038
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	$ 4,606,689	$ 4,320,474
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.73%	14.20%
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 3,283,842	$ 3,194,676
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	$ 3,127,469	$ 3,042,548
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 4,303,872	$ 4,057,111
Leverage Ratio, Actual, Ratio	8.63%	8.10%
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 1,995,630	$ 2,003,152
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 2,494,538	$ 2,503,940
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%