Regulatory Matters (Actual and Required Capital Ratios) (Detail) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one to risk-weighted assets, actual, capital amount	$ 2,239,186	$ 1,986,200
Common equity tier one to risk-weighted assets, actual, ratio	12.52%	11.37%
Common equity tier one to risk-weighted assets, minimum capital required basel III phase-in schedule, capital amount	$ 916,360	$ 786,344
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio	5.125%	4.50%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,251,425	$ 1,222,837
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	$ 1,162,213	$ 1,135,830
CommonEquityTierOneCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets, Ratio	6.50%	6.50%
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,669,717	$ 2,419,545
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.93%	13.85%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount	$ 1,542,168	$ 1,397,944
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio	8.625%	8.00%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,877,137	$ 1,834,256
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	$ 1,788,020	$ 1,747,430
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,383,672	$ 2,163,936
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.33%	12.38%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount	$ 1,184,563	$ 1,048,458
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio	6.625%	6.00%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,519,587	$ 1,484,874
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	$ 1,430,416	$ 1,397,944
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	8.00%	8.00%
Leverage Ratio, Actual, Capital Amount	$ 2,383,672	$ 2,163,936
Leverage Ratio, Actual, Ratio	8.14%	7.79%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount	$ 1,171,682	$ 1,111,325
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	$ 1,171,573	$ 1,111,117
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	$ 1,464,602	$ 1,389,156
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	$ 2,296,480	$ 2,131,360
Common equity tier one to risk-weighted assets, actual, ratio	12.88%	12.24%
Common equity tier one to risk-weighted assets, minimum capital required basel III phase-in schedule, capital amount	$ 913,460	$ 783,727
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio	5.125%	4.50%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,247,463	$ 1,218,766
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	$ 1,158,535	$ 1,132,049
CommonEquityTierOneCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets, Ratio	6.50%	6.50%
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,449,525	$ 2,267,219
Total Capital to Risk-Weighted Assets, Actual, Ratio	13.74%	13.02%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount	$ 1,537,286	$ 1,393,292
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio	8.625%	8.00%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,871,194	$ 1,828,149
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	$ 1,782,361	$ 1,741,615
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,296,480	$ 2,131,360
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.88%	12.24%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount	$ 1,180,814	$ 1,044,969
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio	6.625%	6.00%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,514,776	$ 1,479,930
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	$ 1,425,889	$ 1,393,292
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	8.00%	8.00%
Leverage Ratio, Actual, Capital Amount	$ 2,296,480	$ 2,131,360
Leverage Ratio, Actual, Ratio	7.85%	7.68%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount	$ 1,170,249	$ 1,110,143
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	$ 1,170,141	$ 1,109,935
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	$ 1,462,812	$ 1,387,679
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%