Regulatory Matters (Actual And Required Capital Ratios) (Details) (USD $) In Thousands, unless otherwise specified	Mar. 31, 2012	Dec. 31, 2011
Cullen/Frost [Member]
Regulatory Assets [Line Items]
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 1,844,944	$ 1,835,428
Total Capital to Risk-Weighted Assets, Actual, Ratio	16.10%	16.24%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	916,676	903,970
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	1,145,845	1,129,962
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	1,657,763	1,625,281
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	14.47%	14.38%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	458,338	451,985
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	687,507	677,977
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	6.00%	6.00%
Leverage Ratio, Actual, Capital Amount	1,657,763	1,625,281
Leverage Ratio, Actual, Ratio	8.68%	8.66%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	764,255	750,643
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	955,319	938,304
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%
Frost Bank [Member]
Regulatory Assets [Line Items]
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	1,657,936	1,641,921
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.48%	14.54%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	916,245	903,427
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	1,145,307	1,129,284
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	1,550,755	1,531,774
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.54%	13.56%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	458,123	451,713
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	687,184	677,570
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	6.00%	6.00%
Leverage Ratio, Actual, Capital Amount	1,550,755	1,531,774
Leverage Ratio, Actual, Ratio	8.12%	8.17%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	763,906	750,249
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 954,882	$ 937,811
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%