Regulatory Matters (Actual and Required Capital Ratios) (Detail) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019	Dec. 31, 2018
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Common equity tier one to risk-weighted assets, actual, capital amount	$ 2,857,250		$ 2,642,475
Common equity tier one to risk-weighted assets, actual, ratio	12.36%		12.27%
Common equity tier one to risk-weighted assets, minimum capital required basel III phase-in schedule, capital amount			$ 1,372,573
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio		4.50%	6.375%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,617,886		$ 1,507,139
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio	7.00%		7.00%
Common Equity Tier One Capital Required To Be Well Capitalized, Capital Amount	$ 1,502,323		$ 1,399,486
CommonEquityTierOneCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets, Ratio	6.50%		6.50%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 3,001,736		$ 2,786,961
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.99%		12.94%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount			$ 1,695,532
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio			7.875%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,964,576		$ 1,830,098
Tier One Risk Based Capital Required for Capital Adequacy 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	$ 1,849,013		$ 1,722,445
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	$ 3,367,403		$ 3,152,593
Total Capital to Risk-Weighted Assets, Actual, Ratio	14.57%	8.00%	14.64%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount			$ 2,126,143
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio			9.875%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 2,426,829		$ 2,260,709
Capital Required for Capital Adequacy 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	$ 2,311,266		$ 2,153,056
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%		10.00%
Leverage Ratio, Actual, Capital Amount	$ 3,001,736		$ 2,786,961
Leverage Ratio, Actual, Ratio	9.28%		9.06%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount			$ 1,231,028
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Phase In Schedule, Ratio			4.00%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,293,188		$ 1,231,028
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Fully Phased In, Ratio	4.00%		4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 1,616,485		$ 1,538,785
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 one to risk-weighted assets, actual, capital amount	$ 2,958,326		$ 2,743,973
Common equity tier one to risk-weighted assets, actual, ratio	12.82%		12.78%
Common equity tier one to risk-weighted assets, minimum capital required basel III phase-in schedule, capital amount			$ 1,368,701
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio		4.50%	6.375%
Common Equity Tier One Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,615,206		$ 1,502,887
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Fully Phased In, Ratio	7.00%		7.00%
Common Equity Tier One Capital Required To Be Well Capitalized, Capital Amount	$ 1,499,834		$ 1,395,538
CommonEquityTierOneCapitalRequiredToBeWellCapitalizedToRiskWeightedAssets, Ratio	6.50%		6.50%
Tier 1 Capital to Risk-Weighted Assets, Actual, Capital Amount	$ 2,958,326		$ 2,743,973
Tier 1 Capital to Risk-Weighted Assets, Actual, Ratio	12.82%		12.78%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount			$ 1,690,748
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio			7.875%
Tier One Risk Based Capital Required For Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,961,322		$ 1,824,934
Tier One Risk Based Capital Required for Capital Adequacy 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	$ 1,845,950		$ 1,717,585
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	$ 3,090,993		$ 2,876,605
Total Capital to Risk-Weighted Assets, Actual, Ratio	13.40%	8.00%	13.40%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount			$ 2,120,144
Capital Required for Capital Adequacy to Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio			9.875%
Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 2,422,809		$ 2,254,331
Capital Required for Capital Adequacy 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	$ 2,307,438		$ 2,146,982
Total Capital to Risk-Weighted Assets, Required to be Considered Well Capitalized, Ratio	10.00%		10.00%
Leverage Ratio, Actual, Capital Amount	$ 2,958,326		$ 2,743,973
Leverage Ratio, Actual, Ratio	9.15%		8.93%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Phase In Schedule, Capital Amount			$ 1,229,650
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Phase In Schedule, Ratio			4.00%
Tier One Leverage Capital Required for Capital Adequacy Minimum Capital Required Basel III Fully Phased In, Capital Amount	$ 1,292,743		$ 1,229,650
Tier One Leverage Capital Required for Capital Adequacy to Average Assets Minimum Capital Required Basel III Fully Phased In, Ratio	4.00%		4.00%
Leverage Ratio, Required to be Considered Well Capitalized, Capital Amount	$ 1,615,929		$ 1,537,062
Leverage Ratio, Required to be Considered Well Capitalized, Ratio	5.00%		5.00%