Regulatory Matters (Actual and Required Capital Ratios) (Detail) - USD ($) $ in Thousands	Dec. 31, 2025	Dec. 31, 2024	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	$ 4,601,579	$ 4,343,666
Common Equity Tier One Risk-Based Capital Ratio, Actual	14.06%	13.62%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,291,102	$ 2,232,822
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	$ 4,747,031	$ 4,489,118
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	14.50%	14.07%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,782,053	$ 2,711,283
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	$ 1,963,802	$ 1,913,847
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	$ 5,220,324	$ 4,954,136
Total Capital to Risk-Weighted Assets, Actual, Ratio	15.95%	15.53%	8.00%
Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 3,436,653	$ 3,349,232
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	$ 3,273,003	$ 3,189,745
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 4,747,031	$ 4,489,118
Leverage Ratio, Actual, Ratio	8.80%	8.63%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,157,988	$ 2,079,715
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	$ 4,687,844	$ 4,387,862
Common Equity Tier One Risk-Based Capital Ratio, Actual	14.33%	13.76%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,290,525	$ 2,231,710
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	$ 2,126,916	$ 2,072,302
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	$ 4,687,844	$ 4,387,862
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	14.33%	13.76%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,781,351	$ 2,709,934
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	$ 2,617,742	$ 2,550,526
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	$ 5,021,137	$ 4,692,880
Total Capital to Risk-Weighted Assets, Actual, Ratio	15.34%	14.72%	8.00%
Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 3,435,787	$ 3,347,565
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	$ 3,272,178	$ 3,188,157
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 4,687,844	$ 4,387,862
Leverage Ratio, Actual, Ratio	8.69%	8.44%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,158,485	$ 2,079,965
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	$ 2,698,106	$ 2,599,956
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%