Regulatory Matters (Actual and Required Capital Ratios) (Detail) (USD $) In Thousands, unless otherwise specified	Dec. 31, 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	$ 2,325,818	$ 2,110,774
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.55%	15.52%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	1,278,797	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	1,598,496	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	2,186,276	1,958,336
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.68%	14.39%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	639,398	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	959,098	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	2,186,276	1,958,336
Leverage Ratio, Actual, Ratio	8.16%	8.49%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	1,072,035	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	1,340,043	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	2,071,012	1,780,313
Total Capital to Risk-Weighted Assets, Actual, Ratio	12.99%	13.12%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	1,275,858	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	1,594,823	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	1,979,415	1,707,307
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.41%	12.58%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	637,929	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	956,894	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	1,979,415	1,707,307
Leverage Ratio, Actual, Ratio	7.40%	7.42%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	1,070,109	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	$ 1,337,636	$ 1,150,134
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%