Capital and Regulatory Matters (Narrative) (Detail)					12 Months Ended
	Jan. 24, 2024 USD ($)	Jan. 25, 2023 USD ($)	Nov. 19, 2020 USD ($) $ / shares shares	Mar. 17, 2017 USD ($)	Dec. 31, 2023 USD ($) quarter $ / shares Rate shares	Dec. 31, 2022 USD ($) $ / shares Rate shares	Dec. 31, 2021 USD ($) shares	Jan. 01, 2019 Rate
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit loss optional transitional period under CECL					5 years
CECL Transition Amount					$ 30,800,000	$ 46,200,000
Subordinated notes					$ 99,491,000	$ 99,335,000
Treasury stock, shares, acquired | shares					436,765	0	0
Treasury Stock, Value, Acquired, Cost Method					$ (42,720,000)	$ (4,391,000)	$ (3,864,000)
Excise Tax Percentage | Rate					1.00%
Aggregate dividends available for payment without prior regulatory approval					$ 1,100,000,000
Maximum number of quarterly periods by which the corporation has the right to defer interest payments on junior subordinated deferrable interest debentures | quarter					20
Series B Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Non-cumulative perpetual preferred stock, dividend rate percentage			4.45%
Preferred Stock, Shares Issued | shares			150,000		150,000	150,000
Preferred Stock, Aggregate Liquidation Preference, Value			$ 150,000,000		$ 1,000	$ 1,000
Preferred Stock, Par or Stated Value Per Share | $ / shares			$ 0.01		$ 0.01	$ 0.01
Liquidation preference per share | $ / shares			$ 1,000
Number of Preferred Stock equivalent depositary shares | shares			40
Payments of Stock Issuance Costs			$ 4,500,000
Proceeds from issuance of preferred stock			$ 145,500,000
Series B Preferred Stock Equivalent Depositary Shares Member [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Liquidation preference per share | $ / shares			$ 25
Subordinated Debt [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Subordinated notes				$ 100,000,000
Subordinated notes interest rate				4.50%
Per year percentage of decreases in permissible portion of Subordinate Note included in Total Capital					20.00%
Final years of the term of the notes					5 years
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Additional Tier 1 Capital beyond Common Equity Tier 1					$ 145,500,000	$ 145,500,000
Non-cumulative perpetual preferred stock, dividend rate percentage			4.45%
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio								4.50%
Capital Conservation Buffer								2.50%
Effective common equity tier one capital to risk weighted assets, ratio upon full implementation					7.00%	7.00%		7.00%
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets | Rate								6.00%
Effective tier one capital ratio, upon full implementation					8.50%	8.50%		8.50%
Minimum ratio of Total capital to risk weighted assets | Rate					15.18%	14.84%		8.00%
Tier one risk based capital to risk weighted assets, ratio upon full implementation					10.50%	10.50%		10.50%
Minimum leverage ratio					4.00%	4.00%		4.00%
Cullen/Frost [Member] | Trust Preferred Securities Tier Two Capital Allowable Portion [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust preferred securities					$ 120,000,000	$ 120,000,000
Cullen/Frost [Member] | Subordinated Debt [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Permissible Portion Of Subordinated Note Included In Capital					60,000,000	80,000,000
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Additional Tier 1 Capital beyond Common Equity Tier 1					$ 0	$ 0
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio								4.50%
Capital Conservation Buffer								2.50%
Effective common equity tier one capital to risk weighted assets, ratio upon full implementation					7.00%	7.00%		7.00%
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets | Rate								6.00%
Effective tier one capital ratio, upon full implementation					8.50%	8.50%		8.50%
Minimum ratio of Total capital to risk weighted assets | Rate					14.20%	13.80%		8.00%
Tier one risk based capital to risk weighted assets, ratio upon full implementation					10.50%	10.50%		10.50%
Minimum leverage ratio					4.00%	4.00%		4.00%
stock repurchase 2024 plan | Subsequent Event
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Share repurchase authorized amount	$ 150,000,000
Stock repurchase program, period in force	1 year
stock repurchase 2023 plan
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Share repurchase authorized amount		$ 100,000,000
Stock repurchase program, period in force		1 year
stock repurchase 2023 plan | Treasury Stock, Common
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares					400,868
Treasury Stock, Value, Acquired, Cost Method					$ (39,000,000)
Share awards vesting | Treasury Stock, Common
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares					35,897	31,351	31,317
Treasury Stock, Value, Acquired, Cost Method					$ (3,500,000)	$ (4,400,000)	$ (3,900,000)