Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands	Jun. 30, 2016	Dec. 31, 2015
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 2,065,635	$ 1,986,200
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	11.90%	11.37%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 889,931	$ 786,344
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	5.125%	4.50%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,215,306	$ 1,222,837
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	$ 1,128,693	$ 1,135,830
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	$ 2,210,121	$ 2,163,936
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.73%	12.38%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,150,399	$ 1,048,458
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	6.625%	6.00%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,475,729	$ 1,484,874
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	$ 1,389,160	$ 1,397,944
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	$ 2,492,835	$ 2,419,545
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.36%	13.85%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,497,689	$ 1,397,944
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio	8.625%	8.00%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,822,960	$ 1,834,256
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	$ 1,736,451	$ 1,747,430
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 2,210,121	$ 2,163,936
Leverage Ratio, Actual, Ratio	8.13%	7.79%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,087,126	$ 1,111,325
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	4.00%	4.00%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,087,007	$ 1,111,117
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	$ 1,358,908	$ 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 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 2,207,967	$ 2,131,360
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	12.76%	12.24%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 886,974	$ 783,727
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	5.125%	4.50%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,211,267	$ 1,218,766
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	$ 1,124,942	$ 1,132,049
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	$ 2,207,967	$ 2,131,360
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.76%	12.24%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,146,576	$ 1,044,969
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	6.625%	6.00%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,470,824	$ 1,479,930
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	$ 1,384,544	$ 1,393,292
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	$ 2,357,681	$ 2,267,219
Total Capital to Risk-Weighted Assets, Actual, Ratio	13.62%	13.02%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,492,712	$ 1,393,292
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio	8.625%	8.00%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,816,901	$ 1,828,149
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	$ 1,730,680	$ 1,741,615
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 2,207,967	$ 2,131,360
Leverage Ratio, Actual, Ratio	8.13%	7.68%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,085,798	$ 1,110,143
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	4.00%	4.00%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,085,678	$ 1,109,935
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	$ 1,357,247	$ 1,387,679
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%