Capital and Regulatory Matters - Additional Information (Detail) $ in Thousands	1 Months Ended	3 Months Ended	6 Months Ended		12 Months Ended
	Feb. 28, 2007	Mar. 31, 2016 USD ($)	Jun. 30, 2015 USD ($) quarter shares	Jun. 30, 2014 USD ($) shares	Dec. 31, 2014 USD ($)	Jan. 01, 2019	Apr. 30, 2015 USD ($)
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Stock Repurchase Program, Authorized Amount							$ 100,000
Stock Repurchase Program, Period in Force			2 years
Treasury Stock, Shares, Acquired | shares			140,571	0
Purchase of treasury stock (140,571 shares in 2015)			$ (10,594)	$ 0
Preferred stock value issued include for regulatory tier one capital calculations			$ 144,500
Preferred stock dividend rate percentage			5.375%
Trust preferred securities			$ 133,000
Corporation's aggregate subordinated notes			137,115		$ 137,115
Maximum dividends available without prior regulatory approval			$ 353,000
Maximum number of quarterly periods by which the corporation has the right to defer interest payments on junior subordinated deferrable interest debentures | quarter			20
Two Thousand And Sixteen [Member] | Scenario, Forecast [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
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
Two Thousand And Nineteen [Member] | Scenario, Forecast [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Minimum ratio of Tier 1 capital to risk-weighted assets						6.00%
Minimum ratio of Total capital to risk-weighted assets						8.00%
Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets						4.50%
Capital Conservation Buffer						2.50%
Two Thousand And Nineteen [Member] | Scenario, Forecast [Member] | Minimum [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Effective Tier 1 Capital Ratio, Upon Full Implementation						8.50%
Tier One Risk Based Capital to Risk Weighted Assets, Ratio Upon Full Implementation						10.50%
Minimum leverage ratio						4.00%
Effective Common Equity Tier One Capital To Risk Weighted Assets, Ratio Upon Full Implementation						7.00%
Trust Preferred Securities Tier One Allowable Portion [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust preferred securities			$ 33,300
Trust Preferred Securities Tier One Excluded Portion [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Trust preferred securities			99,800
Floating Rate Subordinated Notes [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Corporation's aggregate subordinated notes			$ 100,000
Per Year Percentage decrease in permissible portion of subordinate note included in Total Capital			20.00%
Final years of the term of the notes			5 years
Debt Instrument, Maturity Date	Feb. 15, 2017
Permissible portion of subordinated note included in total capital			$ 20,000		$ 40,000
Floating Rate Subordinated Notes [Member] | Two Thousand And Sixteen [Member] | Scenario, Forecast [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Permissible portion of subordinated note included in total capital		$ 0
Cullen/Frost [Member]
Compliance with Regulatory Capital Requirements under Banking Regulations [Line Items]
Minimum ratio of Tier 1 capital to risk-weighted assets					4.00%
Minimum ratio of Total capital to risk-weighted assets			14.06%		14.55%
Minimum leverage ratio			8.07%		8.16%
Common Equity Tier 1 Risk Based Capital To Risk Weighted Assets			4.50%