Capital and Regulatory Matters - Additional Information (Detail)					3 Months Ended			6 Months Ended		12 Months Ended
	Mar. 16, 2020 USD ($) shares	Jan. 01, 2020 USD ($)	Jul. 24, 2019 USD ($)	Feb. 15, 2013 USD ($)	Mar. 31, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares	Jun. 30, 2019 USD ($) shares	Jun. 30, 2020 USD ($) quarter shares	Jun. 30, 2019 USD ($) shares	Dec. 31, 2019 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock dividend rate percentage	5.375%
Cumulative Effect on Retained Earnings, Net of Tax		$ (29,300,000)
CECL Transition Amount								$ (61,100,000)
Estimated Incremental Effect of CECL								$ (31,900,000)
Redemption of preferred stock (6,000,000 shares) | shares	6,000,000							6,000,000
Stock Repurchase Program, Authorized Amount			$ 100,000,000.0
Stock Repurchase Program, Period in Force			1 year
Treasury Stock, Shares, Acquired | shares							496,307	181,825	501,658
Treasury Stock, Value, Acquired, Cost Method							$ (50,000,000)	$ (13,973,000)	$ (50,519,000)
Proceeds from Issuance of Preferred Stock and Preference Stock				$ 5,514,000
Preferred stock, redemption amount	$ 150,000,000.0							(150,000,000)
Maximum dividends available without prior regulatory approval								$ 451,700,000
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									$ 25
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock dividend rate percentage								5.375%		5.375%
Additional Tier 1 Capital beyond Common Equity Tier 1								$ 0
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		$ 133,000,000
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 2019 plan [Member] | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury Stock, Shares, Acquired | shares					177,834	202,724
Treasury Stock, Value, Acquired, Cost Method					$ (13,700,000)	$ (17,200,000)
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
Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Proceeds from Issuance of Preferred Stock and Preference Stock				$ 144,486,000
Preferred stock, redemption amount	144,500,000							(144,486,000)
Retained Earnings [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock, redemption amount	$ 5,500,000							$ (5,514,000)