Regulatory Matters (Actual and Required Capital Ratios) (Details) (USD $) In Thousands, unless otherwise specified	Jun. 30, 2011	Dec. 31, 2010
Cullen/Frost [Member]
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 1,784,505	$ 1,720,691
Total Capital to Risk-Weighted Assets, Actual, Ratio	16.42%	15.91%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	869,328	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 Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount
Total Capital to Risk-Weighted Assets, Required to be Well Capitalized Under Prompt Corrective Action Regulations, Ratio
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	1,561,794	1,494,375
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	14.37%	13.82%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	434,664	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 Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount
Tier 1 Capital to Risk-Weighted Assets, Required to be Well Capitalized Under Prompt Corrective Action Regulations, Ratio
Leverage Ratio, Actual, Capital Amount	1,561,794	1,494,375
Leverage Ratio, Actual, Ratio	8.94%	8.68%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	698,459	688,880
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount
Leverage Ratio, Required to be Well Capitalized Under Prompt Corrective Action Regulations, Ratio
Frost Bank [Member]
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	1,608,202	1,558,977
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.81%	14.43%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	868,684	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 Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount	1,085,856	1,080,397
Total Capital to Risk-Weighted Assets, Required to be Well Capitalized Under Prompt Corrective Action Regulations, Ratio	10.00%	10.00%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	1,485,461	1,432,661
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.68%	13.26%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	434,342	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 Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount	651,513	648,238
Tier 1 Capital to Risk-Weighted Assets, Required to be Well Capitalized Under Prompt Corrective Action Regulations, Ratio	6.00%	6.00%
Leverage Ratio, Actual, Capital Amount	1,485,461	1,432,661
Leverage Ratio, Actual, Ratio	8.51%	8.33%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	697,921	688,196
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Well Capitalized Under Prompt Corrective Action Regulations, Capital Amount	$ 872,401	$ 860,246
Leverage Ratio, Required to be Well Capitalized Under Prompt Corrective Action Regulations, Ratio	5.00%	5.00%