Capital and Regulatory Matters (Narrative) (Detail)						3 Months Ended		12 Months Ended
	Jan. 26, 2022 USD ($)	Nov. 19, 2020 USD ($) $ / shares shares	Mar. 16, 2020 USD ($) $ / shares	Mar. 17, 2017 USD ($)	Feb. 15, 2013 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) $ / shares Rate shares	Mar. 31, 2020 USD ($) shares	Dec. 31, 2021 USD ($) quarter $ / shares Rate shares	Dec. 31, 2020 USD ($) $ / shares Rate shares	Dec. 31, 2019 USD ($) shares	Dec. 31, 2004 USD ($)	Jan. 01, 2020 USD ($)	Jan. 01, 2019 USD ($) Rate	Dec. 31, 2018 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit loss optional transitional period under CECL								5 years
Payments for Repurchase of Trust Preferred Securities								$ 13,403,000
Subordinated notes						$ 99,178,000		$ 99,178,000	$ 99,021,000
Cumulative effect of accounting change										$ (29,252,000)				$ (14,672,000)
Credit loss transitional period in years under CECL								3 years
CECL Transition Amount						61,600,000		$ 61,600,000
Estimated Incremental Effect of CECL								$ 32,400,000
Stock Redeemed or Called During Period, Value									$ 150,000,000
Treasury stock, shares, acquired | shares								31,317	206,951	716,062
Treasury Stock, Value, Acquired, Cost Method								$ (3,864,000)	$ (15,785,000)	$ (68,793,000)
Aggregate dividends available for payment without prior regulatory approval						$ 494,100,000		$ 494,100,000
Maximum number of quarterly periods by which the corporation has the right to defer interest payments on junior subordinated deferrable interest debentures | quarter								20
Subsequent Event
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Share repurchase authorized amount	$ 100,000,000
Stock repurchase program, period in force	1 year
WNB Capital Trust I [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust preferred securities											$ 13,000,000
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)
Stock Redeemed or Called During Period, Value									$ 5,514,000
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
Preferred Stock, Aggregate Liquidation Preference, Value					$ 150,000,000
Preferred Stock, Par or Stated Value Per Share | $ / shares					$ 0.01
Liquidation preference per share | $ / shares			$ 25		$ 25
Payments of Stock Issuance Costs					$ 5,500,000
Proceeds from issuance of preferred stock					$ 144,500,000
Stock Redeemed or Called During Period, Value			$ 150,000,000
Series B Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Non-cumulative perpetual preferred stock, dividend rate percentage		4.45%
Preferred Stock, Shares Issued | shares		150,000				150,000		150,000	150,000
Preferred Stock, Aggregate Liquidation Preference, Value		$ 150,000,000				$ 1,000		$ 1,000	$ 1,000
Preferred Stock, Par or Stated Value Per Share | $ / shares		$ 0.01				$ 0.01		$ 0.01	$ 0.01
Liquidation preference per share | $ / shares		$ 1,000
Number of Preferred Stock equivalent depositary shares | shares		40
Payments of Stock Issuance Costs		$ 4,500,000
Proceeds from issuance of preferred stock		$ 145,500,000
Series B Preferred Stock Equivalent Depositary Shares Member [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Liquidation preference per share | $ / shares		$ 25
Subordinated Debt [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Subordinated notes				$ 100,000,000
Subordinated notes interest rate				4.50%
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]
Additional Tier 1 Capital beyond Common Equity Tier 1						$ 145,500,000		$ 145,500,000	$ 145,500,000
Non-cumulative perpetual preferred stock, dividend rate percentage		4.45%
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio													4.50%
Capital Conservation Buffer													2.50%
Effective common equity tier one capital to risk weighted assets, ratio upon full implementation						7.00%		7.00%	7.00%				7.00%
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets | Rate													6.00%
Effective tier one capital ratio, upon full implementation						8.50%		8.50%	8.50%				8.50%
Minimum ratio of Total capital to risk weighted assets | Rate						15.45%		15.45%	15.44%				8.00%
Tier one risk based capital to risk weighted assets, ratio upon full implementation						10.50%		10.50%	10.50%				10.50%
Minimum leverage ratio						4.00%		4.00%	4.00%				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								$ 120,000,000	$ 133,000,000
Cullen/Frost [Member] | Trust Preferred Securities Tier Two Capital Allowable Portion [Member] | WNB Capital Trust I [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Payments for Repurchase of Trust Preferred Securities						$ 13,000,000
Cullen/Frost [Member] | Subordinated Debt [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Permissible Portion Of Subordinated Note Included In Capital						100,000,000		100,000,000	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	$ 0
Common Equity Tier One Risk Based Capital To Risk Weighted Assets Minimum Capital Required Basel III Phase In Schedule, Ratio													4.50%
Capital Conservation Buffer													2.50%
Effective common equity tier one capital to risk weighted assets, ratio upon full implementation						7.00%		7.00%	7.00%
Tier One Risk Based Capital Required for Capital Adequacy to Risk Weighted Assets | Rate													6.00%
Effective tier one capital ratio, upon full implementation						8.50%		8.50%	8.50%				8.50%
Minimum ratio of Total capital to risk weighted assets | Rate						13.61%		13.61%	13.76%				8.00%
Tier one risk based capital to risk weighted assets, ratio upon full implementation						10.50%		10.50%	10.50%				10.50%
Minimum leverage ratio						4.00%		4.00%	4.00%				4.00%
Prior stock repurchase plans [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury Stock, Value, Acquired, Cost Method							$ (13,700,000)			$ (67,200,000)
Prior stock repurchase plans [Member] | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares							177,834			699,031
stock repurchase 2021 plan | Treasury Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury stock, shares, acquired | shares								0