Regulatory Matters (Actual and Required Capital Ratios) (Detail) - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022	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	$ 4,036,945	$ 3,751,200
Common Equity Tier One Risk-Based Capital Ratio, Actual	13.25%	12.85%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,132,516	$ 2,042,876
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	$ 4,182,397	$ 3,896,652
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.73%	13.35%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,589,484	$ 2,480,635
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	$ 1,827,871	$ 1,751,036
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	$ 4,625,760	$ 4,330,982
Total Capital to Risk-Weighted Assets, Actual, Ratio	15.18%	14.84%	8.00%
Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 3,198,774	$ 3,064,313
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	$ 3,046,452	$ 2,918,394
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 4,182,397	$ 3,896,652
Leverage Ratio, Actual, Ratio	8.35%	7.29%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,003,020	$ 2,136,680
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	$ 4,057,111	$ 3,789,056
Common Equity Tier One Risk-Based Capital Ratio, Actual	13.33%	13.00%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,129,784	$ 2,040,388
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	$ 1,977,656	$ 1,894,646
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	$ 4,057,111	$ 3,789,056
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.33%	13.00%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,586,166	$ 2,477,614
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	$ 2,434,038	$ 2,331,872
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	$ 4,320,474	$ 4,023,386
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.20%	13.80%	8.00%
Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 3,194,676	$ 3,060,583
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	$ 3,042,548	$ 2,914,841
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 4,057,111	$ 3,789,056
Leverage Ratio, Actual, Ratio	8.10%	7.09%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,003,152	$ 2,136,316
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	$ 2,503,940	$ 2,670,395
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%