Capital and Regulatory Matters - Actual and Required Capital Ratios (Detail) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 2,385,585	$ 2,239,186
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	12.81%	12.52%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,070,444	$ 916,360
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	5.75%	5.125%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,303,367	$ 1,251,425
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased In, Ratio	7.00%	7.00%
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Capital Amount	$ 1,210,067	$ 1,162,213
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Ratio	6.50%	6.50%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,530,071	$ 2,383,672
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	13.59%	13.33%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,349,691	$ 1,184,563
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	7.25%	6.625%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,582,296	$ 1,519,587
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	8.50%	8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,489,314	$ 1,430,416
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	8.00%	8.00%
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,912,629	$ 2,669,717
Total Capital to Risk-Weighted Assets, Actual, Ratio	15.65%	14.93%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,722,019	$ 1,542,168
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio	9.25%	8.625%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,954,601	$ 1,877,137
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	10.50%	10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,861,642	$ 1,788,020
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 2,530,071	$ 2,383,672
Leverage Ratio, Actual, Ratio	8.61%	8.14%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,174,957	$ 1,171,682
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	4.00%	4.00%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,174,910	$ 1,171,573
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 1,468,696	$ 1,464,602
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common Equity Tier 1 to Risk-Weighted Assets, Actual, Capital Amount	$ 2,401,554	$ 2,296,480
Common Equity Tier 1 to Risk-Weighted Assets, Actual , Ratio	12.94%	12.88%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,067,432	$ 913,460
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	5.75%	5.125%
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,299,400	$ 1,247,463
Common Equity Tier 1 to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased In, Ratio	7.00%	7.00%
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Capital Amount	$ 1,206,662	$ 1,158,535
Common Equity Tier 1 to Risk-Weighted Assets, Required To Be Considered Well Capitalized, Ratio	6.50%	6.50%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,401,554	$ 2,296,480
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.94%	12.88%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,345,892	$ 1,180,814
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	7.25%	6.625%
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,577,843	$ 1,514,776
Tier 1 Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	8.50%	8.50%
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,485,123	$ 1,425,889
Tier 1 Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	8.00%	8.00%
Total Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,551,112	$ 2,449,525
Total Capital to Risk-Weighted Assets, Actual, Ratio	13.74%	13.74%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,717,173	$ 1,537,286
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Phase - In Schedule, Ratio	9.25%	8.625%
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,949,100	$ 1,871,194
Total Capital to Risk-Weighted Assets, Minimum Capital Required - Basel III Fully Phased-In, Ratio	10.50%	10.50%
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Capital Amount	$ 1,856,403	$ 1,782,361
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%	10.00%
Leverage Ratio, Actual, Capital Amount	$ 2,401,554	$ 2,296,480
Leverage Ratio, Actual, Ratio	8.18%	7.85%
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Capital Amount	$ 1,173,726	$ 1,170,249
Leverage Ratio, Minimum Capital Required - Basel III Phase-In Schedule, Ratio	4.00%	4.00%
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Capital Amount	$ 1,173,679	$ 1,170,141
Leverage Ratio, Minimum Capital Required - Basel III Fully Phased-In, Ratio	4.00%	4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 1,467,157	$ 1,462,812
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%	5.00%