Regulatory Matters (Actual and Required Capital Ratios) (Detail) - USD ($) $ in Thousands	Dec. 31, 2015	Dec. 31, 2014
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one to risk-weighted assets, actual, capital amount	$ 1,986,200
Common equity tier one to risk-weighted assets, actual, ratio	11.37%
Common equity tier one to risk-weighted assets, minimum capital required basel III phase-in schedule, capital amount	$ 786,344
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule	4.50%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In	$ 1,222,837
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In	7.00%
Common Equity Tier One Capital Required To Be Well Capitalized	$ 1,135,830
CommonEquityTierOneCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets	6.50%
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,419,545	$ 2,325,818
Total Capital to Risk-Weighted Assets, Actual, Ratio	13.85%	14.55%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule	$ 1,397,944
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule	8.00%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In	$ 1,834,256
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In	10.50%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 1,278,797
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio		8.00%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,747,430	$ 1,598,496
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	$ 2,163,936	$ 2,186,276
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.38%	13.68%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Phase In Schedule	$ 1,048,458
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule	6.00%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In	$ 1,484,874
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In	8.50%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 639,398
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio		4.00%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,397,944	$ 959,098
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	8.00%	6.00%
Leverage Ratio, Actual, Capital Amount	$ 2,163,936	$ 2,186,276
Leverage Ratio, Actual, Ratio	7.79%	8.16%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule	$ 1,111,325
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Phase In Schedule	4.00%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In	$ 1,111,117
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Fully Phased In	4.00%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 1,072,035
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Ratio		4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 1,389,156	$ 1,340,043
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	$ 2,131,360
Common equity tier one to risk-weighted assets, actual, ratio	12.24%
Common equity tier one to risk-weighted assets, minimum capital required basel III phase-in schedule, capital amount	$ 783,727
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule	4.50%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In	$ 1,218,766
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In	7.00%
Common Equity Tier One Capital Required To Be Well Capitalized	$ 1,132,049
CommonEquityTierOneCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets	6.50%
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,267,219	$ 2,071,012
Total Capital to Risk-Weighted Assets, Actual, Ratio	13.02%	12.99%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule	$ 1,393,292
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule	8.00%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In	$ 1,828,149
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In	10.50%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 1,275,858
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio		8.00%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,741,615	$ 1,594,823
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	$ 2,131,360	$ 1,979,415
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.24%	12.41%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Phase In Schedule	$ 1,044,969
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule	6.00%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In	$ 1,479,930
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In	8.50%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 637,929
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio		4.00%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,393,292	$ 956,894
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	8.00%	6.00%
Leverage Ratio, Actual, Capital Amount	$ 2,131,360	$ 1,979,415
Leverage Ratio, Actual, Ratio	7.68%	7.40%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule	$ 1,110,143
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Phase In Schedule	4.00%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In	$ 1,109,935
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Fully Phased In	4.00%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 1,070,109
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Ratio		4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 1,387,679	$ 1,337,636
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%