Capital and Regulatory Matters (Narrative) (Detail)						12 Months Ended
	Nov. 19, 2020 USD ($) $ / shares shares	Mar. 16, 2020 USD ($) $ / shares	Jul. 24, 2019 USD ($)	Mar. 17, 2017 USD ($)	Feb. 15, 2013 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) quarter $ / shares Rate shares	Dec. 31, 2019 USD ($) $ / shares Rate shares	Dec. 31, 2018 USD ($) shares	Jan. 01, 2020 USD ($)	Jan. 01, 2019 USD ($) Rate	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Credit loss transitional period in years under CECL						5 years
Subordinated notes						$ 99,021,000	$ 98,865,000
Cumulative effect of accounting change							$ (29,252,000)	$ (14,672,000)				$ (2,285,000)
CECL Transition Amount						63,700,000
Estimated Incremental Effect of CECL						34,400,000
Stock Redeemed or Called During Period, Value						$ 150,000,000
Share repurchase authorized amount			$ 100,000,000.0
Stock repurchase program, period in force			1 year
Treasury stock, shares, acquired | shares						206,951	716,062	1,037,982
Treasury Stock, Value, Acquired, Cost Method						$ (15,785,000)	$ (68,793,000)	$ (101,010,000)
Aggregate dividends available for payment without prior regulatory approval						$ 503,900,000
Maximum number of quarterly periods by which the corporation has the right to defer interest payments on junior subordinated deferrable interest debentures | quarter						20
Preferred Stock [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock Redeemed or Called During Period, Value						$ 144,486,000
Retained Earnings [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Cumulative effect of accounting change							$ (29,252,000)	(14,672,000)	$ (29,300,000)	$ (14,700,000)	$ (2,300,000)	$ (2,285,000)
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		6,000,000
Preferred Stock, Aggregate Liquidation Preference, Value					$ 150,000,000.0		$ 25
Preferred Stock, Par or Stated Value Per Share | $ / shares					$ 0.01		$ 0.01
Liquidation preference per share | $ / shares		$ 25			$ 25
Payments of Stock Issuance Costs					$ 5,500,000
Stock Redeemed or Called During Period, Value		$ 150,000,000.0
Proceeds from issuance of preferred stock					$ 144,500,000
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
Preferred Stock, Aggregate Liquidation Preference, Value	$ 150,000,000.0					$ 1,000
Preferred Stock, Par or Stated Value Per Share | $ / shares	$ 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]
Non-cumulative perpetual preferred stock						$ 145,500,000	$ 144,500,000
Non-cumulative perpetual preferred stock, dividend rate percentage	4.45%						5.375%
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%
Minimum ratio of Total capital to risk weighted assets | Rate						15.44%	14.57%			8.00%
Tier one risk based capital to risk weighted assets, ratio upon full implementation						10.50%	10.50%			10.50%
Minimum leverage ratio						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						$ 133,000,000.0	$ 133,000,000.0
Cullen/Frost [Member] | Subordinated Debt [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Permissible Portion Of Subordinated Note Included In Capital						100,000,000.0	100,000,000.0
Frost Bank [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Additional Tier 1 Capital beyond Common Equity Tier 1						$ 0	$ 0
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%
Minimum ratio of Total capital to risk weighted assets | Rate						13.76%	13.40%			8.00%
Tier one risk based capital to risk weighted assets, ratio upon full implementation						10.50%	10.50%			10.50%
Minimum leverage ratio						4.00%	4.00%			4.00%
stock repurchase 2019 plan [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Treasury Stock, Value, Acquired, Cost Method						$ (13,700,000)	$ (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						177,834	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