Regulatory Matters (Actual And Required Capital Ratios) (Details) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2011	Dec. 31, 2010
Cullen/Frost [Member]
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 1,806,869	$ 1,720,691
Total Capital to Risk-Weighted Assets, Actual, Ratio	16.57%	15.91%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	872,423	865,081
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	1,090,529	1,081,351
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	1,591,436	1,494,375
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	14.59%	13.82%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	436,212	432,540
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	654,317	648,811
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	6.00%	6.00%
Leverage Ratio, Actual, Capital Amount	1,591,436	1,494,375
Leverage Ratio, Actual, Ratio	8.82%	8.68%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	721,670	688,880
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized , Capital Amount	902,088	861,100
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%
Frost Bank [Member]
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	1,622,983	1,558,977
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.89%	14.43%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	871,896	864,318
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	1,089,870	1,080,397
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	1,507,550	1,432,661
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.83%	13.26%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	435,948	432,159
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	653,922	648,238
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	6.00%	6.00%
Leverage Ratio, Actual, Capital Amount	1,507,550	1,432,661
Leverage Ratio, Actual, Ratio	8.36%	8.33%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	721,150	688,196
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized , Capital Amount	$ 901,438	$ 860,246
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%