Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands	Sep. 30, 2023	Dec. 31, 2022
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 3,977,860	$ 3,751,200
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	13.32%	12.85%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,090,045	$ 2,042,876
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	$ 4,123,312	$ 3,896,652
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.81%	13.35%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,537,911	$ 2,480,635
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	$ 1,791,467	$ 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	$ 4,561,577	$ 4,330,982
Total Capital to Risk-Weighted Assets, Actual, Ratio	15.28%	14.84%
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 3,135,067	$ 3,064,313
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	$ 2,985,778	$ 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	$ 4,123,312	$ 3,896,652
Leverage Ratio, Actual, Ratio	8.17%	7.29%
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,018,632	$ 2,136,680
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	$ 4,013,794	$ 3,789,056
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	13.46%	13.00%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,087,478	$ 2,040,388
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	$ 1,938,372	$ 1,894,646
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	$ 4,013,794	$ 3,789,056
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.46%	13.00%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,534,795	$ 2,477,614
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	$ 2,385,689	$ 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	$ 4,272,059	$ 4,023,386
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.33%	13.80%
Total Capital to Risk-Weighted Assets, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 3,131,217	$ 3,060,583
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	$ 2,982,112	$ 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	$ 4,013,794	$ 3,789,056
Leverage Ratio, Actual, Ratio	7.96%	7.09%
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Capital Amount	$ 2,018,241	$ 2,136,316
Leverage Ratio, Minimum Capital Required Plus Capital Conservation Buffer, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 2,522,801	$ 2,670,395
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%