Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - Scenario, Unspecified [Domain] - Range [Domain] - USD ($) $ in Thousands	Jun. 30, 2015	Dec. 31, 2014
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 1,996,899
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	11.70%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 768,340
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	4.50%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,194,771
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased In, Ratio	7.00%
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Capital Amount	$ 1,109,824
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Ratio	6.50%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,174,635	$ 2,185,558
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.74%	13.67%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,024,453
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	6.00%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,450,793
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,365,937	$ 959,098
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	8.00%	6.00%
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,400,992	$ 2,325,100
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.06%	14.55%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,365,937
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio	8.00%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,792,156
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,707,421	$ 1,598,496
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 2,174,635	$ 2,185,558
Leverage Ratio, Actual, Ratio	8.07%	8.16%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,077,693
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	4.00%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,077,451
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Ratio	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 1,347,116	$ 1,340,043
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 639,398
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio		4.00%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 1,278,797
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio		8.00%
Leverage Ratio, Minimum Capital Required for Capital Adequacy Purposes, Capital Amount		$ 1,072,035
Leverage Ratio, Minimum Capital Required for Capital Adequacy Purposes, Ratio		4.00%
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 2,064,681
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	12.13%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 765,916
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	4.50%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,191,002
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased In, Ratio	7.00%
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Capital Amount	$ 1,106,324
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Ratio	6.50%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,064,681	$ 1,978,697
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.13%	12.41%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,021,222
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	6.00%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,446,216
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,361,629	$ 956,894
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	8.00%	6.00%
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,171,288	$ 2,070,293
Total Capital to Risk-Weighted Assets, Actual, Ratio	12.76%	12.98%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,361,629
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio	8.00%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,786,503
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,702,037	$ 1,594,823
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 2,064,681	$ 1,978,697
Leverage Ratio, Actual, Ratio	7.67%	7.40%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,076,815
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	4.00%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,076,573
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Ratio	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 1,346,019	$ 1,337,636
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 637,929
Tier 1 Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio		4.00%
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Capital Amount		$ 1,275,858
Total Capital to Risk-Weighted Assets, Minimum Required for Capital Adequacy Purposes, Ratio		8.00%
Leverage Ratio, Minimum Capital Required for Capital Adequacy Purposes, Capital Amount		$ 1,070,109
Leverage Ratio, Minimum Capital Required for Capital Adequacy Purposes, Ratio		4.00%