Capital and Regulatory Matters - Additional Information (Detail) $ / shares in Units, $ in Thousands			3 Months Ended		6 Months Ended		12 Months Ended
	Jan. 28, 2026 USD ($)	Nov. 19, 2020 USD ($)	Jun. 30, 2026 USD ($) $ / shares shares	Jun. 30, 2025 USD ($) shares	Jun. 30, 2026 USD ($) quarter $ / shares shares	Jun. 30, 2025 USD ($) shares	Dec. 31, 2025 USD ($) $ / shares shares
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury Stock, Shares, Acquired | shares			654,955	606	1,174,053	19,033
Purchase of treasury stock			$ 90,895	$ 76	$ 163,193	$ 2,676
Maximum dividends available without prior regulatory approval			$ 772,500		$ 772,500
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]
Preferred stock, shares issued | shares			150,000		150,000		150,000
Preferred Stock, Aggregate Liquidation Preference, Value			$ 150,000		$ 150,000
Preferred stock, par value | $ / shares			$ 0.01		$ 0.01		$ 0.01
Preferred Stock, Liquidation Preference Per Share | $ / shares			$ 1,000		$ 1,000		$ 1,000
Number of Preferred Stock equivalent depositary shares | shares			40		40
Payments of Stock Issuance Costs		$ 4,500
Proceeds from Issuance of Preferred Stock and Preference Stock		$ 145,500
Series B Preferred Stock Equivalent Depositary Shares Member
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred Stock, Liquidation Preference Per Share | $ / shares			$ 25		$ 25
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Additional Tier 1 Capital beyond Common Equity Tier 1			$ 145,500		$ 145,500		$ 145,500
Preferred stock dividend rate percentage					4.45%		4.45%
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		$ 120,000
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Additional Tier 1 Capital beyond Common Equity Tier 1			0		$ 0		$ 0
prior stock repurchase plans | Treasury Stock, Common
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury Stock, Shares, Acquired | shares						0
stock repurchase 2026 plan
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Share Repurchase Program, Authorized, Amount	$ 300,000
Share Repurchase Program, Period in Force	1 year
stock repurchase 2026 plan | Treasury Stock, Common
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury Stock, Shares, Acquired | shares					1,162,708
Purchase of treasury stock					$ 160,000
Share awards vesting | Treasury Stock, Common
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury Stock, Shares, Acquired | shares					11,345	19,033
Purchase of treasury stock					$ 1,600	$ 2,700
Subordinated Debt [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Permissible Percentage Decreased Each Of Final Five Years Of Term Of Notes					20.00%		20.00%
Final years of the term of the notes					5 years		5 years
Subordinated Debt [Member] | Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Permissible portion of subordinated note included in capital			$ 0		$ 0		$ 20,000