Capital and Regulatory Matters (Narrative) (Detail)					1 Months Ended	12 Months Ended
	Jul. 24, 2019 USD ($)	Mar. 17, 2017 USD ($)	Jan. 01, 2016	Feb. 15, 2013 USD ($) $ / shares shares	Feb. 28, 2007 USD ($)	Dec. 31, 2019 USD ($) quarter $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) shares	Jan. 01, 2019
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Subordinated notes						$ 98,865,000	$ 98,708,000
Preferred stock, shares issued | shares						6,000,000	6,000,000
Preferred stock, par value | $ / shares						$ 0.01	$ 0.01
Proceeds from issuance of preferred stock				$ 144,500,000
Share repurchase authorized amount	$ 100,000,000.0
Stock repurchase program, period in force	1 year
Treasury stock, shares, acquired | shares						716,062	1,037,982	1,149,555
Treasury Stock, Value, Acquired, Cost Method						$ (68,793,000)	$ (101,010,000)	$ (101,473,000)
Aggregate dividends available for payment without prior regulatory approval						$ 682,900,000
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]
Non-cumulative perpetual preferred stock, dividend rate percentage				5.375%
Preferred stock, shares issued | shares				6,000,000
Series A Preferred Stock, Liquidation preference value				$ 150,000,000.0		$ 25	25
Preferred stock, par value | $ / shares				$ 0.01
Liquidation preference per share | $ / shares				$ 25
Subordinated Debt [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Subordinated notes		$ 100,000,000			$ 100,000,000
Subordinated notes interest rate		4.50%			5.75%
Per year percentage of decreases in permissible portion of Subordinate Note included in Total Capital						20.00%
Final years of the term of the notes						5 years
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Non-cumulative perpetual preferred stock						$ 144,500,000	$ 144,500,000
Non-cumulative perpetual preferred stock, dividend rate percentage						5.375%	5.375%
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio							6.375%		4.50%
Capital Conservation Buffer									2.50%
Effective common equity tier one capital to risk weighted assets, ratio upon full implementation									7.00%
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets									6.00%
Effective tier one capital ratio, upon full implementation									8.50%
Minimum ratio of Total capital to risk weighted assets						14.57%	14.64%		8.00%
Minimum leverage ratio						9.28%	9.06%
Tier one capital conservation buffer to risk weighted assets required under regulatory			0.625%
Tier one risk based capital conservation buffer initial requirements phasing duration			4 years
Cullen/Frost [Member] | Minimum [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier one risk based capital to risk weighted assets, ratio upon full implementation									10.50%
Minimum leverage ratio									4.00%
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
Cullen/Frost [Member] | Subordinated Debt [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Permissible portion of subordinated note included in total capital						100,000,000.0	100,000,000
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Additional Tier 1 Capital beyond Common Equity Tier 1						$ 0	$ 0
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio							6.375%		4.50%
Capital Conservation Buffer									2.50%
Effective common equity tier one capital to risk weighted assets, ratio upon full implementation									7.00%
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets									6.00%
Effective tier one capital ratio, upon full implementation									8.50%
Minimum ratio of Total capital to risk weighted assets						13.40%	13.40%		8.00%
Minimum leverage ratio						9.15%	8.93%
Tier one capital conservation buffer to risk weighted assets required under regulatory			0.625%
Tier one risk based capital conservation buffer initial requirements phasing duration			4 years
Frost Bank [Member] | Minimum [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Tier one risk based capital to risk weighted assets, ratio upon full implementation									10.50%
Minimum leverage ratio									4.00%
stock repurchase 2019 plan [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury Stock, Value, Acquired, Cost Method						$ (17,200,000)
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
stock repurchase 2017 plan [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury Stock, Value, Acquired, Cost Method						$ (50,000,000.0)	$ (100,000,000.0)
stock repurchase 2017 plan [Member] | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares						496,307	1,027,292
stock repurchase 2016 plan [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury Stock, Value, Acquired, Cost Method								$ (100,000,000.0)
stock repurchase 2016 plan [Member] | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares								1,134,966