Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2013	Dec. 31, 2012
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 1,967,401	$ 1,947,974
Total Capital to Risk-Weighted Assets, Actual, Ratio	15.44%	15.11%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	1,019,653	1,031,526
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	1,274,566	1,289,408
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	1,813,812	1,763,521
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	14.23%	13.68%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	509,826	515,763
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	764,739	773,645
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	6.00%	6.00%
Leverage Ratio, Actual, Capital Amount	1,813,812	1,763,521
Leverage Ratio, Actual, Ratio	8.42%	8.28%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	861,796	851,483
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	1,077,245	1,064,354
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	1,740,052	1,730,444
Total Capital to Risk-Weighted Assets, Actual, Ratio	13.66%	13.43%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	1,019,045	1,030,878
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	1,273,806	1,288,597
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	1,646,463	1,625,991
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.93%	12.62%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	509,522	515,439
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	764,283	773,158
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	6.00%	6.00%
Leverage Ratio, Actual, Capital Amount	1,646,463	1,625,991
Leverage Ratio, Actual, Ratio	7.65%	7.64%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	861,162	850,954
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 1,076,453	$ 1,063,693
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%