Capital and Regulatory Matters - Additional Information (Detail) $ / shares in Units, $ in Millions			3 Months Ended	6 Months Ended		12 Months Ended
	Jan. 26, 2022 USD ($)	Nov. 19, 2020 USD ($)	Mar. 31, 2022	Jun. 30, 2022 USD ($) quarter $ / shares shares	Jun. 30, 2021 shares	Dec. 31, 2021 USD ($) $ / shares shares
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
CECL Transition Amount				$ 46.2		$ 61.6
Treasury Stock, Shares, Acquired | shares				7,459	11,625
Maximum dividends available without prior regulatory approval				$ 447.3
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				$ 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 2022 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 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				$ 80.0		$ 100.0