Capital and Regulatory Matters - Additional Information (Detail) $ in Thousands		3 Months Ended			12 Months Ended
	Oct. 24, 2017 USD ($)	Mar. 31, 2019 USD ($) quarter shares	Dec. 31, 2018 USD ($) shares	Mar. 31, 2018 shares	Dec. 31, 2018 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock Repurchase Program, Authorized Amount	$ 150,000
Stock Repurchase Program, Period in Force	2 years
Treasury stock, shares, acquired | shares		5,351		0
Treasury Stock, Value, Acquired, Cost Method		$ (519)
Qualified subordinated debt		98,747	$ 98,708		$ 98,708
Maximum dividends available without prior regulatory approval		$ 521,500
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
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
Stock Repurchase 2017 Plan | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares		0	1,027,292
Treasury Stock, Value, Acquired, Cost Method			$ (100,000)
Subordinated Debt [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Qualified subordinated debt		$ 100,000	$ 100,000		$ 100,000