Regulatory Matters (Actual and Required Capital Ratios) (Detail) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one to risk-weighted assets, actual, capital amount	$ 2,642,475	$ 2,426,048
Common equity tier one to risk-weighted assets, actual, ratio	12.65%	12.42%
Common equity tier one to risk-weighted assets, minimum capital required basel III phase-in schedule, capital amount	$ 1,332,052	$ 1,123,430
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio	6.375%	5.75%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,462,645	$ 1,367,583
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio	7.00%	7.00%
Common Equity Tier One Capital Required To Be Well Capitalized, Capital Amount	$ 1,358,171	$ 1,269,965
CommonEquityTierOneCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets, Ratio	6.50%	6.50%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,786,961	$ 2,570,534
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.34%	13.16%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount	$ 1,645,476	$ 1,416,499
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio	7.875%	7.25%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,776,069	$ 1,660,637
Tier One Risk Based Capital Required for Capital Adequacy 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	$ 1,671,595	$ 1,563,033
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	$ 3,152,593	$ 2,959,326
Total Capital to Risk-Weighted Assets, Actual, Ratio	15.09%	15.15%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount	$ 2,063,375	$ 1,807,257
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio	9.875%	9.25%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 2,193,968	$ 2,051,375
Capital Required for Capital Adequacy 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	$ 2,089,494	$ 1,953,792
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 2,786,961	$ 2,570,534
Leverage Ratio, Actual, Ratio	9.06%	8.46%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount	$ 1,231,028	$ 1,215,227
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Phase In Schedule, Ratio	4.00%	4.00%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,231,028	$ 1,215,186
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Fully Phased In, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 1,538,785	$ 1,519,034
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one to risk-weighted assets, actual, capital amount	$ 2,743,973	$ 2,518,999
Common equity tier one to risk-weighted assets, actual, ratio	13.17%	12.92%
Common equity tier one to risk-weighted assets, minimum capital required basel III phase-in schedule, capital amount	$ 1,328,180	$ 1,120,663
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio	6.375%	5.75%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,458,393	$ 1,364,214
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio	7.00%	7.00%
Common Equity Tier One Capital Required To Be Well Capitalized, Capital Amount	$ 1,354,222	$ 1,266,836
CommonEquityTierOneCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets, Ratio	6.50%	6.50%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,743,973	$ 2,518,999
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.17%	12.92%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount	$ 1,640,693	$ 1,413,010
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio	7.875%	7.25%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,770,906	$ 1,656,546
Tier One Risk Based Capital Required for Capital Adequacy 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	$ 1,666,735	$ 1,559,183
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	$ 2,876,605	$ 2,674,791
Total Capital to Risk-Weighted Assets, Actual, Ratio	13.81%	13.72%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount	$ 2,057,376	$ 1,802,805
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio	9.875%	9.25%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 2,187,590	$ 2,046,321
Capital Required for Capital Adequacy 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	$ 2,083,419	$ 1,948,979
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 2,743,973	$ 2,518,999
Leverage Ratio, Actual, Ratio	8.93%	8.30%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount	$ 1,229,650	$ 1,214,295
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Phase In Schedule, Ratio	4.00%	4.00%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,229,650	$ 1,214,254
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Fully Phased In, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 1,537,062	$ 1,517,869
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%