Capital and Regulatory Matters - Additional Information (Detail)				3 Months Ended			9 Months Ended		12 Months Ended
	Mar. 16, 2020 USD ($) shares	Jul. 24, 2019 USD ($)	Feb. 15, 2013 USD ($)	Sep. 30, 2020 USD ($) shares	Mar. 31, 2020 USD ($) shares	Sep. 30, 2019 USD ($) shares	Sep. 30, 2020 USD ($) quarter shares	Sep. 30, 2019 USD ($) shares	Dec. 31, 2019 USD ($)	Jan. 01, 2020 USD ($)	Jan. 01, 2019 USD ($)	Dec. 31, 2018 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Cumulative effect of accounting change									$ (29,252,000)			$ (14,672,000)
CECL Transition Amount				$ (63,700,000)			$ (63,700,000)
Estimated Incremental Effect of CECL							$ (34,400,000)
Proceeds from Issuance of Preferred Stock and Preference Stock			$ 5,500,000
Stock Redeemed or Called During Period, 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
Stock Repurchase Program, Authorized Amount		$ 100,000,000.0
Stock Repurchase Program, Period in Force		1 year
Treasury Stock, Shares, Acquired | shares				400		202,724	182,225	704,382
Treasury Stock, Value, Acquired, Cost Method				$ (27,000)		$ (17,190,000)	$ (14,000,000)	$ (67,709,000)
Maximum dividends available without prior regulatory approval				482,100,000			$ 482,100,000
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]
Additional Tier 1 Capital beyond Common Equity Tier 1				0			$ 0		144,500,000
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		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			100,000,000.0		100,000,000.0
Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Proceeds from Issuance of Preferred Stock and Preference Stock			$ 144,500,000
Preferred stock, redemption amount	144,500,000						144,486,000
Retained Earnings [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Cumulative effect of accounting change									$ (29,252,000)	$ (29,300,000)	$ (14,700,000)	$ (14,672,000)
Preferred stock, redemption amount	$ 5,500,000						$ 5,514,000