Capital and Regulatory Matters - Additional Information (Detail)					3 Months Ended	6 Months Ended		12 Months Ended
	Jan. 27, 2021 USD ($)	Nov. 19, 2020 USD ($) $ / shares shares	Mar. 16, 2020 USD ($) shares	Feb. 15, 2013 USD ($)	Mar. 31, 2020 USD ($) shares	Jun. 30, 2021 USD ($) quarter $ / shares shares	Jun. 30, 2020 USD ($) shares	Dec. 31, 2020 USD ($) $ / shares shares
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
CECL Transition Amount						$ 62,800,000		$ 63,700,000
Redemption of series A preferred stock (6,000,000 shares) | shares			6,000,000				6,000,000
Preferred stock dividend rate percentage			5.375%
Preferred stock, redemption amount			$ 150,000,000.0				$ 150,000,000
Proceeds from Issuance of Preferred Stock and Preference Stock				$ 5,500,000
Purchase of treasury stock (11,625 shares during 2021 and 181,825 shares during 2020) | shares						11,625	181,825
Treasury Stock, Value, Acquired, Cost Method						$ (1,288,000)	$ (13,973,000)
Maximum dividends available without prior regulatory approval						$ 396,200,000
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 A Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock liquidation preference value			25
Series B Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock dividend rate percentage		4.45%
Preferred stock liquidation preference value		$ 150,000,000.0				$ 1,000		$ 1,000
Preferred stock, shares issued | shares		150,000				150,000		150,000
Preferred stock, par value | $ / shares		$ 0.01				$ 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						$ 133,000,000.0		$ 133,000,000.0
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Additional Tier 1 Capital beyond Common Equity Tier 1						$ 0		0
stock repurchase 2019 plan [Member] | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Purchase of treasury stock (11,625 shares during 2021 and 181,825 shares during 2020) | shares					177,834
Treasury Stock, Value, Acquired, Cost Method					$ (13,700,000)
stock repurchase 2021 plan
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock Repurchase Program, Authorized Amount	$ 100,000,000.0
Stock Repurchase Program, Period in Force	1 year
stock repurchase 2021 plan | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Purchase of treasury stock (11,625 shares during 2021 and 181,825 shares during 2020) | shares						0
Subordinated Debt [Member] | Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Permissible portion of subordinated note included in capital						$ 100,000,000.0		$ 100,000,000.0
Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock, redemption amount			144,500,000				144,486,000
Proceeds from Issuance of Preferred Stock and Preference Stock				$ 144,500,000
Retained Earnings [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock, redemption amount			$ 5,500,000				$ 5,514,000