Capital and Regulatory Matters - Additional Information (Detail)			3 Months Ended		12 Months Ended
	Jan. 26, 2022 USD ($)	Nov. 19, 2020 USD ($)	Mar. 31, 2022 USD ($) quarter $ / shares shares	Mar. 31, 2021 shares	Dec. 31, 2021 USD ($) $ / shares shares
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
CECL Transition Amount			$ 46,200,000		$ 61,600,000
Preferred Stock, Aggregate Liquidation Preference, Value			$ 150,000,000
Treasury Stock, Shares, Acquired | shares			7,459	11,625
Maximum dividends available without prior regulatory approval			$ 325,000,000
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]
Preferred stock dividend rate percentage		4.45%
Preferred stock, shares issued | shares			150,000		150,000
Preferred Stock, Aggregate Liquidation Preference, Value			$ 1,000		$ 1,000
Preferred stock, par value | $ / shares			$ 0.01		$ 0.01
Preferred Stock, 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 and Preference Stock		$ 145,500,000
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			$ 145,500,000		$ 145,500,000
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			$ 120,000,000		$ 120,000,000
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Additional Tier 1 Capital beyond Common Equity Tier 1			$ 0		0
stock repurchase 2022 plan
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock Repurchase Program, Authorized Amount	$ 100,000,000
Stock Repurchase Program, Period in Force	1 year
stock repurchase 2022 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%
Final years of the term of the notes			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			$ 80,000,000		$ 100,000,000