Regulatory Matters (Actual And Required Capital Ratios) (Details) (USD $) In Thousands, unless otherwise specified	Dec. 31, 2011	Dec. 31, 2010
Cullen/Frost [Member]
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 1,835,428	$ 1,720,691
Total Capital to Risk-Weighted Assets, Actual, Ratio	16.24%	15.91%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	903,970	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	1,129,962	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	1,625,281	1,494,375
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	14.38%	13.82%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	451,985	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	677,977	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	1,625,281	1,494,375
Leverage Ratio, Actual, Ratio	8.66%	8.68%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	750,643	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	938,304	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	1,641,921	1,558,977
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.54%	14.43%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	903,427	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	1,129,284	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	1,531,774	1,432,661
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.56%	13.26%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	451,713	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	677,570	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	1,531,774	1,432,661
Leverage Ratio, Actual, Ratio	8.17%	8.33%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	750,249	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	$ 937,811	$ 860,246
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%