Capital and Regulatory Matters - Additional Information (Detail) $ / shares in Units, $ in Millions			3 Months Ended		12 Months Ended
	Jan. 25, 2023 USD ($)	Nov. 19, 2020 USD ($)	Mar. 31, 2023 USD ($) quarter $ / shares shares	Mar. 31, 2022 shares	Dec. 31, 2022 USD ($) $ / shares shares
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
CECL Transition Amount			$ 30.8		$ 46.2
Treasury Stock, Shares, Acquired | shares			9,183	7,459
Maximum dividends available without prior regulatory approval			$ 873.7
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
Preferred Stock, Aggregate Liquidation Preference, Value			$ 150.0
Preferred stock, par value | $ / shares			$ 0.01		$ 0.01
Preferred Stock, Liquidation Preference Per Share | $ / shares			$ 1,000		$ 1,000
Number of Preferred Stock equivalent depositary shares | shares			40
Payments of Stock Issuance Costs		$ 4.5
Proceeds from Issuance of Preferred Stock and Preference Stock		$ 145.5
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
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Additional Tier 1 Capital beyond Common Equity Tier 1			$ 145.5		$ 145.5
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			$ 120.0		$ 120.0
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Additional Tier 1 Capital beyond Common Equity Tier 1			$ 0.0		$ 0.0
stock repurchase 2023 plan
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock Repurchase Program, Authorized Amount	$ 100.0
Stock Repurchase Program, Period in Force	1 year
stock repurchase 2023 plan | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury Stock, Shares, Acquired | shares			0
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			$ 60.0		$ 80.0