Borrowed Funds	12 Months Ended
Dec. 31, 2015
Debt Disclosure [Abstract]
Borrowed Funds	Borrowed Funds Federal Funds Purchased and Securities Sold Under Agreements to Repurchase. Federal funds purchased are short-term borrowings that typically mature within one to ninety days. Federal funds purchased totaled $9.6 million and $12.0 million at December 31, 2015 and 2014. Securities sold under agreements to repurchase are secured short-term borrowings that typically mature within thirty to ninety days. Securities sold under agreements to repurchase are stated at the amount of cash received in connection with the transaction. We may be required to provide additional collateral based on the fair value of the underlying securities. Securities sold under agreements to repurchase totaled $883.9 million and $791.1 million at December 31, 2015 and 2014. Subordinated Notes Payable. In February 2007, we issued $100 million of 5.75% fixed-to-floating rate subordinated notes that mature on February 15, 2017. The notes, which qualified as Tier 2 capital for Cullen/Frost under the capital rules in effect prior to 2015 (see Note 10 - Capital and Regulatory Matters), had an interest rate of 5.75% per annum, payable semi-annually on each February 15 and August 15, commencing on August 15, 2007 until February 15, 2012. From February 15, 2012, to but excluding the maturity date or date of earlier redemption and commencing on May 15, 2012, the notes bear interest at a rate per annum equal to three-month LIBOR for the related interest period plus 0.53% (0.89% and 0.76% at December 31, 2015 and 2014), payable quarterly on each February 15, May 15, August 15 and November 15. The notes are subordinated in right of payment to all of our senior indebtedness and effectively subordinated to all existing and future debt and all other liabilities of our subsidiaries. The notes cannot be accelerated except in the event of bankruptcy or the occurrence of certain other events of bankruptcy, insolvency or reorganization. We may elect to redeem the notes (subject to regulatory approval), in whole or in part, on any interest payment date on or after February 15, 2012 at a redemption price equal to 100% of the principal amount plus any accrued and unpaid interest. Unamortized debt issuance costs related to these notes, which are included in other assets, totaled $130 thousand and $250 thousand at December 31, 2015 and 2014. Proceeds from sale of the notes were used to fund a portion of the redemption of certain junior subordinated deferrable interest debentures. Junior Subordinated Deferrable Interest Debentures. At December 31, 2015 and 2014, we had $123.7 million of junior subordinated deferrable interest debentures issued to Cullen/Frost Capital Trust II (“Trust II”), a wholly owned Delaware statutory business trust. Unamortized debt issuance costs related to Trust II, which are included in other assets, totaled $1.0 million and $1.1 million at December 31, 2015 and 2014. At December 31, 2015 and 2014, we also had $13.4 million of junior subordinated deferrable interest debentures issued to WNB Capital Trust I (“WNB Trust”), a wholly owned Delaware statutory business trust acquired in connection with the acquisition of WNB during the second quarter of 2014. Trust II and WNB Trust are variable interest entities for which we are not the primary beneficiary. As such, the accounts of Trust II and WNB Trust are not included in our consolidated financial statements. See Note 1 - Summary of Significant Accounting Policies for additional information about our consolidation policy. Details of our transactions with the capital trust are presented below. Trust II was formed in 2004 for the purpose of issuing $120 million of floating rate (three-month LIBOR plus a margin of 1.55%) trust preferred securities, which represent beneficial interests in the assets of the trust. The trust preferred securities will mature on March 1, 2034 and are redeemable with the approval of the Federal Reserve Board in whole or in part at our option at any time after March 1, 2009 and in whole at any time upon the occurrence of certain events affecting their tax or regulatory capital treatment. Distributions on the trust preferred securities are payable quarterly in arrears on March 1, June 1, September 1 and December 1 of each year. Trust II also issued $3.7 million of common equity securities to Cullen/Frost. The proceeds of the offering of the trust preferred securities and common equity securities were used to purchase $123.7 million of floating rate (three-month LIBOR plus a margin of 1.55%, which was equal to 1.96% and 1.78% at December 31, 2015 and 2014) junior subordinated deferrable interest debentures issued by us, which have terms substantially similar to the trust preferred securities. In October 2008, we entered into an interest rate swap contract on the junior subordinated deferrable interest debentures that effectively fixed the interest rate on the debentures for a period of five years, terminating in October 2013. See Note 16 - Derivative Financial Instruments. WNB Trust was formed in 2004 by WNB for the purpose of issuing $13.0 million of floating rate (three-month LIBOR plus a margin of 2.35%) trust preferred securities, which represent beneficial interests in the assets of the trust. The trust preferred securities will mature on July 23, 2034 and are redeemable with the approval of the Federal Reserve Board in whole or in part at our option at any time after July 23, 2009 and in whole at any time upon the occurrence of certain events affecting their tax or regulatory capital treatment. Distributions on the trust preferred securities are payable quarterly in arrears on January 23, April 23, July 23 and October 23 of each year. WNB Trust also issued $403 thousand of common equity securities to WNB. The proceeds of the offering of the trust preferred securities and common equity securities were used to purchase $13.4 million of floating rate (three-month LIBOR plus a margin of 2.35%, which was equal to 2.67% and 2.58% at December 31, 2015and 2014) junior subordinated deferrable interest debentures issued by WNB, which have terms substantially similar to the trust preferred securities. We have the right at any time during the term of the debentures issued to Trust II and WNB Trust to defer payments of interest at any time or from time to time for an extension period not exceeding 20 consecutive quarterly periods with respect to each extension period. Under the terms of the debentures, in the event that under certain circumstances there is an event of default under the debentures or we have elected to defer interest on the debentures, we may not, with certain exceptions, declare or pay any dividends or distributions on our capital stock or purchase or acquire any of our capital stock. Payments of distributions on the trust preferred securities and payments on redemption of the trust preferred securities are guaranteed by us on a limited basis. We are obligated by agreement to pay any costs, expenses or liabilities of Trust II and WNB Trust other than those arising under the trust preferred securities. Our obligations under the junior subordinated debentures, the related indentures, the trust agreements establishing the trusts, the guarantees and the agreements as to expenses and liabilities, in the aggregate, constitute a full and unconditional guarantee by us of Trust II’s and WNB Trust's obligations under the trust preferred securities. Although the accounts of Trust II and WNB Trust are not included in our consolidated financial statements, the $120.0 million in trust preferred securities issued by Trust II and the $13.0 million in trust preferred securities issued by WNB Trust are included in the capital of Cullen/Frost for regulatory capital purposes as of December 31, 2015 and 2014. See Note 10 - Capital and Regulatory Matters.

Borrowed Funds

Federal Funds Purchased and Securities Sold Under Agreements to Repurchase. Federal funds purchased are short-term borrowings that typically mature within one to ninety days. Federal funds purchased totaled $9.6 million and $12.0 million at December 31, 2015 and 2014. Securities sold under agreements to repurchase are secured short-term borrowings that typically mature within thirty to ninety days. Securities sold under agreements to repurchase are stated at the amount of cash received in connection with the transaction. We may be required to provide additional collateral based on the fair value of the underlying securities. Securities sold under agreements to repurchase totaled $883.9 million and $791.1 million at December 31, 2015 and 2014.

Subordinated Notes Payable. In February 2007, we issued $100 million of 5.75% fixed-to-floating rate subordinated notes that mature on February 15, 2017. The notes, which qualified as Tier 2 capital for Cullen/Frost under the capital rules in effect prior to 2015 (see Note 10 - Capital and Regulatory Matters), had an interest rate of 5.75% per annum, payable semi-annually on each February 15 and August 15, commencing on August 15, 2007 until February 15, 2012. From February 15, 2012, to but excluding the maturity date or date of earlier redemption and commencing on May 15, 2012, the notes bear interest at a rate per annum equal to three-month LIBOR for the related interest period plus 0.53% (0.89% and 0.76% at December 31, 2015 and 2014), payable quarterly on each February 15, May 15, August 15 and November 15. The notes are subordinated in right of payment to all of our senior indebtedness and effectively subordinated to all existing and future debt and all other liabilities of our subsidiaries. The notes cannot be accelerated except in the event of bankruptcy or the occurrence of certain other events of bankruptcy, insolvency or reorganization. We may elect to redeem the notes (subject to regulatory approval), in whole or in part, on any interest payment date on or after February 15, 2012 at a redemption price equal to 100% of the principal amount plus any accrued and unpaid interest. Unamortized debt issuance costs related to these notes, which are included in other assets, totaled $130 thousand and $250 thousand at December 31, 2015 and 2014. Proceeds from sale of the notes were used to fund a portion of the redemption of certain junior subordinated deferrable interest debentures.

Junior Subordinated Deferrable Interest Debentures. At December 31, 2015 and 2014, we had $123.7 million of junior subordinated deferrable interest debentures issued to Cullen/Frost Capital Trust II (“Trust II”), a wholly owned Delaware statutory business trust. Unamortized debt issuance costs related to Trust II, which are included in other assets, totaled $1.0 million and $1.1 million at December 31, 2015 and 2014. At December 31, 2015 and 2014, we also had $13.4 million of junior subordinated deferrable interest debentures issued to WNB Capital Trust I (“WNB Trust”), a wholly owned Delaware statutory business trust acquired in connection with the acquisition of WNB during the second quarter of 2014. Trust II and WNB Trust are variable interest entities for which we are not the primary beneficiary. As such, the accounts of Trust II and WNB Trust are not included in our consolidated financial statements. See Note 1 - Summary of Significant Accounting Policies for additional information about our consolidation policy. Details of our transactions with the capital trust are presented below.

Trust II was formed in 2004 for the purpose of issuing $120 million of floating rate (three-month LIBOR plus a margin of 1.55%) trust preferred securities, which represent beneficial interests in the assets of the trust. The trust preferred securities will mature on March 1, 2034 and are redeemable with the approval of the Federal Reserve Board in whole or in part at our option at any time after March 1, 2009 and in whole at any time upon the occurrence of certain events affecting their tax or regulatory capital treatment. Distributions on the trust preferred securities are payable quarterly in arrears on March 1, June 1, September 1 and December 1 of each year. Trust II also issued $3.7 million of common equity securities to Cullen/Frost. The proceeds of the offering of the trust preferred securities and common equity securities were used to purchase $123.7 million of floating rate (three-month LIBOR plus a margin of 1.55%, which was equal to 1.96% and 1.78% at December 31, 2015 and 2014) junior subordinated deferrable interest debentures issued by us, which have terms substantially similar to the trust preferred securities. In October 2008, we entered into an interest rate swap contract on the junior subordinated deferrable interest debentures that effectively fixed the interest rate on the debentures for a period of five years, terminating in October 2013. See Note 16 - Derivative Financial Instruments.

WNB Trust was formed in 2004 by WNB for the purpose of issuing $13.0 million of floating rate (three-month LIBOR plus a margin of 2.35%) trust preferred securities, which represent beneficial interests in the assets of the trust. The trust preferred securities will mature on July 23, 2034 and are redeemable with the approval of the Federal Reserve Board in whole or in part at our option at any time after July 23, 2009 and in whole at any time upon the occurrence of certain events affecting their tax or regulatory capital treatment. Distributions on the trust preferred securities are payable quarterly in arrears on January 23, April 23, July 23 and October 23 of each year. WNB Trust also issued $403 thousand of common equity securities to WNB. The proceeds of the offering of the trust preferred securities and common equity securities were used to purchase $13.4 million of floating rate (three-month LIBOR plus a margin of 2.35%, which was equal to 2.67% and 2.58% at December 31, 2015and 2014) junior subordinated deferrable interest debentures issued by WNB, which have terms substantially similar to the trust preferred securities.

We have the right at any time during the term of the debentures issued to Trust II and WNB Trust to defer payments of interest at any time or from time to time for an extension period not exceeding 20 consecutive quarterly periods with respect to each extension period. Under the terms of the debentures, in the event that under certain circumstances there is an event of default under the debentures or we have elected to defer interest on the debentures, we may not, with certain exceptions, declare or pay any dividends or distributions on our capital stock or purchase or acquire any of our capital stock.

Payments of distributions on the trust preferred securities and payments on redemption of the trust preferred securities are guaranteed by us on a limited basis. We are obligated by agreement to pay any costs, expenses or liabilities of Trust II and WNB Trust other than those arising under the trust preferred securities. Our obligations under the junior subordinated debentures, the related indentures, the trust agreements establishing the trusts, the guarantees and the agreements as to expenses and liabilities, in the aggregate, constitute a full and unconditional guarantee by us of Trust II’s and WNB Trust's obligations under the trust preferred securities.

Although the accounts of Trust II and WNB Trust are not included in our consolidated financial statements, the $120.0 million in trust preferred securities issued by Trust II and the $13.0 million in trust preferred securities issued by WNB Trust are included in the capital of Cullen/Frost for regulatory capital purposes as of December 31, 2015 and 2014. See Note 10 - Capital and Regulatory Matters.