Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2014	Dec. 31, 2013
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,272,967	$ 2,110,774
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.81%	15.52%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	1,228,072	1,088,349
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio	8.00%	8.00%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	1,535,090	1,360,437
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	2,134,655	1,958,336
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.91%	14.39%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	614,036	544,175
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	921,054	816,262
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	6.00%	6.00%
Leverage Ratio, Actual, Capital Amount	2,134,655	1,958,336
Leverage Ratio, Actual, Ratio	8.27%	8.49%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	1,031,906	922,728
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	1,289,883	1,153,410
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]
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	2,023,592	1,780,313
Total Capital to Risk-Weighted Assets, Actual, Ratio	13.21%	13.12%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	1,225,704	1,085,447
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio	8.00%	8.00%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	1,532,130	1,356,809
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	1,932,526	1,707,307
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.61%	12.58%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	612,852	542,724
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	919,278	814,085
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	6.00%	6.00%
Leverage Ratio, Actual, Capital Amount	1,932,526	1,707,307
Leverage Ratio, Actual, Ratio	7.50%	7.42%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	1,030,358	920,107
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 1,287,947	$ 1,150,134
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%