Regulatory Matters (Actual and Required Capital Ratios) (Detail) - USD ($) $ in Thousands	Dec. 31, 2024	Dec. 31, 2023	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	$ 4,343,666	$ 4,036,945
Common Equity Tier One Risk-Based Capital Ratio, Actual	13.62%	13.25%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,232,822	$ 2,132,516
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio	7.00%	7.00%	7.00%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 4,489,118	$ 4,182,397
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	14.07%	13.73%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,711,283	$ 2,589,484
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio	8.50%	8.50%	8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,913,847	$ 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	$ 4,954,136	$ 4,625,760
Total Capital to Risk-Weighted Assets, Actual, Ratio	15.53%	15.18%	8.00%
Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 3,349,232	$ 3,198,774
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio	10.50%	10.50%	10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 3,189,745	$ 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	$ 4,489,118	$ 4,182,397
Leverage Ratio, Actual, Ratio	8.63%	8.35%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,079,715	$ 2,003,020
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio	4.00%	4.00%	4.00%
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier One to Risk-Weighted Assets, Actual, Capital Amount	$ 4,387,862	$ 4,057,111
Common Equity Tier One Risk-Based Capital Ratio, Actual	13.76%	13.33%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,231,710	$ 2,129,784
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio	7.00%	7.00%	7.00%
Common Equity Tier One Capital Required To Be Well Capitalized, Capital Amount	$ 2,072,302	$ 1,977,656
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	$ 4,387,862	$ 4,057,111
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.76%	13.33%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,709,934	$ 2,586,166
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio	8.50%	8.50%	8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 2,550,526	$ 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	$ 4,692,880	$ 4,320,474
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.72%	14.20%	8.00%
Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 3,347,565	$ 3,194,676
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio	10.50%	10.50%	10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 3,188,157	$ 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	$ 4,387,862	$ 4,057,111
Leverage Ratio, Actual, Ratio	8.44%	8.10%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,079,965	$ 2,003,152
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Plus Capital Conservation Buffer, Ratio	4.00%	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 2,599,956	$ 2,503,940
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%