Regulatory Matters (Actual And Required Capital Ratios) (Detail) (USD $) In Thousands, unless otherwise specified	Sep. 30, 2012	Dec. 31, 2011
Cullen/Frost [Member]
Regulatory Assets [Line Items]
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 1,912,550	$ 1,835,428
Total Capital to Risk-Weighted Assets, Actual, Ratio	15.62%	16.24%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	979,742	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	1,224,678	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	1,727,149	1,625,281
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	14.10%	14.38%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	489,871	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	734,807	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	1,727,149	1,625,281
Leverage Ratio, Actual, Ratio	8.59%	8.66%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	804,371	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	1,005,464	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	1,705,199	1,641,921
Total Capital to Risk-Weighted Assets, Actual, Ratio	13.93%	14.54%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	979,294	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	1,224,117	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	1,599,798	1,531,774
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.07%	13.56%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount	489,647	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	734,470	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	1,599,798	1,531,774
Leverage Ratio, Actual, Ratio	7.96%	8.17%
Leverage Ratio, Minimum Required for Capital Adequacy Purposes, Capital Amount	803,878	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	$ 1,004,848	$ 937,811
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%