Significant Accounting Policies (Policies)	3 Months Ended
Significant Accounting Policies (Policies)	Mar. 31, 2015
Accounting Policies [Abstract]
Nature of Operations	Nature of Operations. Cullen/Frost Bankers, Inc. (“Cullen/Frost”) is a financial holding company and a bank holding company headquartered in San Antonio, Texas that provides, through our subsidiaries, a broad array of products and services throughout numerous Texas markets. In addition to general commercial and consumer banking, other products and services offered include trust and investment management, investment banking, insurance, brokerage, leasing, treasury management and item processing.
Basis of Presentation	Basis of Presentation. The consolidated financial statements in this Quarterly Report on Form 10-Q include the accounts of Cullen/Frost and all other entities in which Cullen/Frost has a controlling financial interest. All significant intercompany balances and transactions have been eliminated in consolidation. The accounting and financial reporting policies we follow conform, in all material respects, to accounting principles generally accepted in the United States and to general practices within the financial services industry. The consolidated financial statements in this Quarterly Report on Form 10-Q have not been audited by an independent registered public accounting firm, but in the opinion of management, reflect all adjustments necessary for a fair presentation of our financial position and results of operations. All such adjustments were of a normal and recurring nature. The consolidated financial statements have been prepared in accordance with accounting principles generally accepted in the United States for interim financial information and with the instructions to Form 10-Q adopted by the Securities and Exchange Commission (“SEC”). Accordingly, the financial statements do not include all of the information and footnotes required by accounting principles generally accepted in the United States for complete financial statements and should be read in conjunction with our consolidated financial statements, and notes thereto, for the year ended December 31, 2014, included in our Annual Report on Form 10-K filed with the SEC on February 5, 2015 (the “2014 Form 10-K”). Operating results for the interim periods disclosed herein are not necessarily indicative of the results that may be expected for a full year or any future period.
Use of Estimates	Use of Estimates. The preparation of financial statements in conformity with accounting principles generally accepted in the United States requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosure of contingent assets and liabilities at the date of the financial statements. Actual results could differ from those estimates. The allowance for loan losses and the fair values of financial instruments and the status of contingencies are particularly subject to change.
Reclassification, Policy [Policy Text Block]	Reclassifications and Restatements. Certain items in prior financial statements have been reclassified to conform to the current presentation. Additionally, certain items in prior financial statements have been restated to reflect adjustments to initially reported provisional amounts recognized in business combinations so that the prior financial statements are reported as if the adjusted amounts had been known as of the measurement date of the business combination. In that regard, during 2015, we made acquisition valuation adjustments impacting certain assets acquired in connection with the acquisition of WNB Bancshares, Inc. (See Note 2 - Mergers and Acquisitions). As a result of these adjustments, our consolidated balance sheet as of December 31, 2014 reflects a $718 thousand increase in goodwill and a $718 thousand decrease in premises and equipment when compared to previously reported amounts.