Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands	Sep. 30, 2022	Dec. 31, 2021
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 3,606,527	$ 3,371,043
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	12.74%	13.13%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,981,042	$ 1,796,549
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased In, Ratio	7.00%	7.00%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 3,751,979	$ 3,516,495
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.26%	13.70%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,405,550	$ 2,181,523
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	8.50%	8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,698,036	$ 1,539,899
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	$ 4,187,120	$ 3,966,244
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.80%	15.45%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,971,562	$ 2,694,823
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	10.50%	10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 2,830,059	$ 2,566,498
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 3,751,979	$ 3,516,495
Leverage Ratio, Actual, Ratio	7.09%	7.34%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,117,091	$ 1,917,533
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, 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	$ 3,595,393	$ 3,261,532
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	12.71%	12.72%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,979,976	$ 1,795,221
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased In, Ratio	7.00%	7.00%
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Capital Amount	$ 1,838,550	$ 1,666,991
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	$ 3,595,393	$ 3,261,532
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.71%	12.72%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,404,257	$ 2,179,911
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	8.50%	8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 2,262,830	$ 2,051,681
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	$ 3,830,534	$ 3,491,281
Total Capital to Risk-Weighted Assets, Actual, Ratio	13.54%	13.61%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,969,965	$ 2,692,831
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	10.50%	10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 2,828,538	$ 2,564,601
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 3,595,393	$ 3,261,532
Leverage Ratio, Actual, Ratio	6.79%	6.80%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 2,117,519	$ 1,917,679
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 2,646,898	$ 2,397,099
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%