Capital and Regulatory Matters - Additional Information (Detail) $ in Thousands			3 Months Ended				9 Months Ended		12 Months Ended
	Jul. 24, 2019 USD ($)	Oct. 24, 2017 USD ($)	Sep. 30, 2019 USD ($) shares	Jun. 30, 2019 USD ($) shares	Dec. 31, 2018 USD ($) shares	Sep. 30, 2018 shares	Sep. 30, 2019 USD ($) quarter shares	Sep. 30, 2018 USD ($) shares	Dec. 31, 2018 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock Repurchase Program, Authorized Amount	$ 100,000	$ 150,000
Stock Repurchase Program, Period in Force	1 year	2 years
Treasury stock, shares, acquired | shares			202,724			0	704,382	601
Treasury Stock, Value, Acquired, Cost Method			$ (17,190)				$ (67,709)	$ (70)
Qualified subordinated debt			98,826		$ 98,708		98,826		$ 98,708
Maximum dividends available without prior regulatory approval			633,800				$ 633,800
Maximum number of quarterly periods by which the corporation has the right to defer interest payments on junior subordinated deferrable interest debentures | quarter							20
Trust Preferred Securities Tier One Excluded Portion [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust preferred securities							$ 133,000		133,000
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Preferred stock value issued include for regulatory tier one capital calculations			144,500		144,500		$ 144,500		$ 144,500
Preferred stock dividend rate percentage							5.375%		5.375%
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 2017 Plan | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares				496,307	1,027,292
Treasury Stock, Value, Acquired, Cost Method				$ (50,000)	$ (100,000)
Stock Repurchase 2019 plan [Member] | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares			202,724
Treasury Stock, Value, Acquired, Cost Method			$ (17,200)
Subordinated Debt [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Qualified subordinated debt			$ 100,000		$ 100,000		$ 100,000		$ 100,000