Summary of Significant Accounting and Reporting Policies	9 Months Ended
Sep. 30, 2016
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation and Significant Accounting Policies [Text Block]	Summary of Significant Accounting and Reporting Policies Basis of Presentation In this report, the words “the Company,” “we,” “us,” and “our” refer to the combined entities of Southside Bancshares, Inc. and its subsidiaries. The words “Southside” and “Southside Bancshares” refer to Southside Bancshares, Inc. The words “Southside Bank” and “the Bank” refer to Southside Bank. “Omni” refers to OmniAmerican Bancorp, Inc., a bank holding company acquired by Southside on December 17, 2014. “SFG” refers to SFG Finance, LLC (formerly Southside Financial Group, LLC), which was a wholly-owned subsidiary of the Bank that was dissolved in April 2015. The consolidated balance sheet as of September 30, 2016, and the related consolidated statements of income, comprehensive income, changes in shareholders’ equity, cash flows and notes to the financial statements for the three- and nine-month periods ended September 30, 2016 and 2015 are unaudited; in the opinion of management, all adjustments necessary for a fair statement of such financial statements have been included. Such adjustments consisted only of normal recurring items. All significant intercompany accounts and transactions are eliminated in consolidation. The preparation of these consolidated financial statements in conformity with U.S. generally accepted accounting principles (“GAAP”) requires the use of management’s estimates. These estimates are subjective in nature and involve matters of judgment. Actual amounts could differ from these estimates. Certain prior period amounts have been reclassified to conform to current year presentation. In connection with the adoption of ASU 2015-03 “Interest - Imputation of Interest (Subtopic 835-30) - Simplifying the Presentation of Debt Issuance Costs,” that requires unamortized debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, our consolidated balance sheet as of December 31, 2015 reflects a decrease of $80,000 in other assets and long-term debt. On May 5, 2016, our board of directors declared a 5% stock dividend to common stock shareholders of record as of May 31, 2016, which was paid on June 28, 2016. All share data has been adjusted to give retroactive recognition to stock dividends. Interim results are not necessarily indicative of results for a full year. These financial statements should be read in conjunction with the financial statements and notes thereto in our Annual Report on Form 10-K for the year ended December 31, 2015. For a description of our significant accounting and reporting policies, refer to “Note 1- Summary of Significant Accounting and Reporting Policies” in our consolidated financial statements in our Annual Report on Form 10-K for the year ended December 31, 2015. The accounting and reporting policies we follow with respect to our derivative instruments and hedging activities are presented below. Derivative Financial Instruments and Hedging Activities Derivative financial instruments are carried on the consolidated balance sheets as other assets or other liabilities, as applicable, at estimated fair value. The accounting for changes in the fair value (i.e., gains or losses) of a derivative financial instrument is determined by whether it has been designated and qualifies as part of a hedging relationship and, further, by the type of hedging relationship. We present derivative financial instruments at fair value in the consolidated balance sheets on a net basis when a right of offset exists, based on transactions with a single counterparty and any cash collateral paid to and/or received from that counterparty for derivative contracts that are subject to legally enforceable master netting arrangements; however, fair value amounts recognized for derivatives and fair value amounts recognized for the right/obligation to reclaim/return cash collateral are not offset for financial reporting purposes. For derivative instruments that are designated and qualify as cash flow hedges (i.e., hedging the exposure to variability in expected future cash flows that is attributable to a particular risk), the effective portion of the gain or loss on the derivative instrument is reported as a component of other comprehensive income and reclassified into earnings in the same period or periods during which the hedged transaction affects earnings. The remaining gain or loss on the derivative instrument in excess of the cumulative change in the present value of future cash flows of the hedged item (i.e., the ineffective portion), if any, is recognized in current earnings during the period of change. For derivative instruments not designated as hedging instruments, the gain or loss is recognized in current earnings during the period of change. For derivatives designated as hedging instruments at inception, statistical regression analysis is used at inception and for each reporting period thereafter to assess whether the derivative used has been and is expected to be highly effective in offsetting changes in the fair value or cash flows of the hedged item. All components of each derivative instrument’s gain or loss are included in the assessment of hedge effectiveness. Net hedge ineffectiveness or losses are recorded in “other noninterest income” on the consolidated statements of income. Further information on our derivative instruments and hedging activities is included in “Note 10 - Derivative Financial Instruments and Hedging Activities.” Accounting Pronouncements In February 2016, the FASB issued ASU 2016-02, “Leases (Topic 842).” ASU 2016-02 requires a lessee to recognize assets and liabilities for leases with lease terms of more than 12 months. Consistent with current GAAP, the recognition, measurement, and presentation of expenses and cash flows arising from a lease by a lessee primarily will depend on its classification as a finance or operating lease. However, unlike current GAAP which requires only capital leases to be recognized on the balance sheet, the new ASU 2016-02 will require both types of leases to be recognized on the balance sheet. ASU 2016-02 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted. We are currently evaluating the potential impact of the pending adoption of ASU 2016-02 on our consolidated financial statements. In March 2016, the FASB issued ASU 2016-09, “Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting.” ASU 2016-09 simplifies several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification in the statement of cash flows. ASU 2016-09 requires that all excess tax benefits and tax deficiencies be recognized as income tax expense or benefit in the income statement and should be classified along with other income tax cash flows as an operating activity instead of a financing activity as currently required under GAAP. ASU 2016-09 also simplifies accounting for forfeitures by allowing an entity to make an entity-wide accounting policy election either to estimate the number of forfeitures expected to occur or to recognize the effects of forfeitures when they occur in compensation cost. Additionally, cash paid by an employer when directly withholding shares for tax-withholding purposes should be classified as a financing activity, and to qualify for equity classification, an employer can now withhold up to the maximum statutory tax rate instead of the minimum statutory tax rate as currently required by GAAP. ASU 2016-09 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2016. Early adoption is permitted. ASU 2016-09 is not expected to have a significant impact on our consolidated financial statements. In April 2016, the FASB issued ASU 2016-10, “Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing.” ASU 2016-10 clarifies certain aspects of ASU 2014-09 “Revenue from Contracts with Customers (Topic 606)” related to (i) identifying performance obligations and (ii) the licensing implementation guidance. ASU 2016-10 is effective concurrently with ASU 2014-09 which we are required to adopt in the first quarter of fiscal year 2018. Early adoption is permitted. The guidance permits companies to either apply the requirements retrospectively to all prior periods presented, or apply the requirements in the year of adoption, through cumulative adjustment. We are currently evaluating the potential impact of the pending adoption of ASU 2016-10 on our consolidated financial statements and we have not yet identified which transition method will be applied upon adoption. In May 2016, the FASB issued ASU 2016-12, “Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients.” ASU 2016-12 clarifies ASU 2014-09 “Revenue from Contracts with Customers (Topic 606)” guidance on (i) assessing collectability, (ii) presenting sales tax, (iii) measuring non-cash consideration and (iv) certain transition matters. ASU 2016-12 is effective concurrently with ASU 2014-09 which we are required to adopt in the first quarter of fiscal year 2018. Early adoption is permitted. The guidance permits companies to either apply the requirements retrospectively to all prior periods presented, or apply the requirements in the year of adoption, through cumulative adjustment. We are currently evaluating the potential impact of the pending adoption of ASU 2016-12 on our consolidated financial statements. In June 2016, the FASB issued ASU 2016-13, “Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.” ASU 2016-13 introduces an approach based on expected losses to estimate credit losses on certain types of financial instruments. ASU 2016-13 also modifies the impairment model for available-for-sale debt securities and provides for a simplified accounting model for purchased financial assets with credit deterioration since their origination. ASU 2016-13 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. The guidance requires companies to apply the requirements in the year of adoption through cumulative adjustment with some aspects of the update requiring a prospective transition approach. We are currently evaluating the potential impact of the pending adoption of ASU 2016-13 on our consolidated financial statements. In August 2016, the FASB issued ASU 2016-15, “Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (a consensus of the Emerging Issues Task Force).” ASU 2016-15 is intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. ASU 2016-15 addresses eight classification issues related to the statement of cash flows: (i) debt prepayment or debt extinguishment, (ii) settlement of zero-coupon bonds, (iii) contingent consideration payments made after a business combination, (iv) proceeds from the settlement of insurance claims, (v) proceeds from the settlement of corporate-owned life insurance policies, including bank-owned life insurance policies, (vi) distributions received from equity method invitees, (vii) beneficial interest in securitizations transactions, and (viii) separately identifiable cash flows and application of the predominance principle. ASU 2016-15 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2017. Early adoption is permitted. The guidance requires companies to apply the requirements retrospectively to all prior periods presented. If it is impracticable for a company to apply ASU 2016-15 retrospectively, requirements may be applied prospectively as of the earliest date practicable. We are currently evaluating the potential impact of the pending adoption of ASU 2016-15 on our consolidated financial statements.

Summary of Significant Accounting and Reporting Policies

Basis of Presentation

In this report, the words “the Company,” “we,” “us,” and “our” refer to the combined entities of Southside Bancshares, Inc. and its subsidiaries. The words “Southside” and “Southside Bancshares” refer to Southside Bancshares, Inc. The words “Southside Bank” and “the Bank” refer to Southside Bank. “Omni” refers to OmniAmerican Bancorp, Inc., a bank holding company acquired by Southside on December 17, 2014. “SFG” refers to SFG Finance, LLC (formerly Southside Financial Group, LLC), which was a wholly-owned subsidiary of the Bank that was dissolved in April 2015.

The consolidated balance sheet as of September 30, 2016, and the related consolidated statements of income, comprehensive income, changes in shareholders’ equity, cash flows and notes to the financial statements for the three- and nine-month periods ended September 30, 2016 and 2015 are unaudited; in the opinion of management, all adjustments necessary for a fair statement of such financial statements have been included. Such adjustments consisted only of normal recurring items. All significant intercompany accounts and transactions are eliminated in consolidation. The preparation of these consolidated financial statements in conformity with U.S. generally accepted accounting principles (“GAAP”) requires the use of management’s estimates. These estimates are subjective in nature and involve matters of judgment. Actual amounts could differ from these estimates.

Certain prior period amounts have been reclassified to conform to current year presentation. In connection with the adoption of ASU 2015-03 “Interest - Imputation of Interest (Subtopic 835-30) - Simplifying the Presentation of Debt Issuance Costs,” that requires unamortized debt issuance costs related to a recognized debt liability be presented in the balance sheet as a direct deduction from the carrying amount of that debt liability, our consolidated balance sheet as of December 31, 2015 reflects a decrease of $80,000 in other assets and long-term debt.

On May 5, 2016, our board of directors declared a 5% stock dividend to common stock shareholders of record as of May 31, 2016, which was paid on June 28, 2016. All share data has been adjusted to give retroactive recognition to stock dividends.

Interim results are not necessarily indicative of results for a full year. These financial statements should be read in conjunction with the financial statements and notes thereto in our Annual Report on Form 10-K for the year ended December 31, 2015.

For a description of our significant accounting and reporting policies, refer to “Note 1- Summary of Significant Accounting and Reporting Policies” in our consolidated financial statements in our Annual Report on Form 10-K for the year ended December 31, 2015. The accounting and reporting policies we follow with respect to our derivative instruments and hedging activities are presented below.

Derivative Financial Instruments and Hedging Activities

Derivative financial instruments are carried on the consolidated balance sheets as other assets or other liabilities, as applicable, at estimated fair value. The accounting for changes in the fair value (i.e., gains or losses) of a derivative financial instrument is determined by whether it has been designated and qualifies as part of a hedging relationship and, further, by the type of hedging relationship. We present derivative financial instruments at fair value in the consolidated balance sheets on a net basis when a right of offset exists, based on transactions with a single counterparty and any cash collateral paid to and/or received from that counterparty for derivative contracts that are subject to legally enforceable master netting arrangements; however, fair value amounts recognized for derivatives and fair value amounts recognized for the right/obligation to reclaim/return cash collateral are not offset for financial reporting purposes.

For derivative instruments that are designated and qualify as cash flow hedges (i.e., hedging the exposure to variability in expected future cash flows that is attributable to a particular risk), the effective portion of the gain or loss on the derivative instrument is reported as a component of other comprehensive income and reclassified into earnings in the same period or periods during which the hedged transaction affects earnings. The remaining gain or loss on the derivative instrument in excess of the cumulative change in the present value of future cash flows of the hedged item (i.e., the ineffective portion), if any, is recognized in current earnings during the period of change. For derivative instruments not designated as hedging instruments, the gain or loss is recognized in current earnings during the period of change.

For derivatives designated as hedging instruments at inception, statistical regression analysis is used at inception and for each reporting period thereafter to assess whether the derivative used has been and is expected to be highly effective in offsetting changes in the fair value or cash flows of the hedged item. All components of each derivative instrument’s gain or loss are included in the assessment of hedge effectiveness. Net hedge ineffectiveness or losses are recorded in “other noninterest income” on the consolidated statements of income.

Further information on our derivative instruments and hedging activities is included in “Note 10 - Derivative Financial Instruments and Hedging Activities.”

Accounting Pronouncements

In February 2016, the FASB issued ASU 2016-02, “Leases (Topic 842).” ASU 2016-02 requires a lessee to recognize assets and liabilities for leases with lease terms of more than 12 months. Consistent with current GAAP, the recognition, measurement, and presentation of expenses and cash flows arising from a lease by a lessee primarily will depend on its classification as a finance or operating lease. However, unlike current GAAP which requires only capital leases to be recognized on the balance sheet, the new ASU 2016-02 will require both types of leases to be recognized on the balance sheet. ASU 2016-02 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. Early adoption is permitted. We are currently evaluating the potential impact of the pending adoption of ASU 2016-02 on our consolidated financial statements.

In March 2016, the FASB issued ASU 2016-09, “Compensation - Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting.” ASU 2016-09 simplifies several aspects of the accounting for share-based payment transactions, including the income tax consequences, classification of awards as either equity or liabilities, and classification in the statement of cash flows. ASU 2016-09 requires that all excess tax benefits and tax deficiencies be recognized as income tax expense or benefit in the income statement and should be classified along with other income tax cash flows as an operating activity instead of a financing activity as currently required under GAAP. ASU 2016-09 also simplifies accounting for forfeitures by allowing an entity to make an entity-wide accounting policy election either to estimate the number of forfeitures expected to occur or to recognize the effects of forfeitures when they occur in compensation cost. Additionally, cash paid by an employer when directly withholding shares for tax-withholding purposes should be classified as a financing activity, and to qualify for equity classification, an employer can now withhold up to the maximum statutory tax rate instead of the minimum statutory tax rate as currently required by GAAP. ASU 2016-09 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2016. Early adoption is permitted. ASU 2016-09 is not expected to have a significant impact on our consolidated financial statements.

In April 2016, the FASB issued ASU 2016-10, “Revenue from Contracts with Customers (Topic 606): Identifying Performance Obligations and Licensing.” ASU 2016-10 clarifies certain aspects of ASU 2014-09 “Revenue from Contracts with Customers (Topic 606)” related to (i) identifying performance obligations and (ii) the licensing implementation guidance. ASU 2016-10 is effective concurrently with ASU 2014-09 which we are required to adopt in the first quarter of fiscal year 2018. Early adoption is permitted. The guidance permits companies to either apply the requirements retrospectively to all prior periods presented, or apply the requirements in the year of adoption, through cumulative adjustment. We are currently evaluating the potential impact of the pending adoption of ASU 2016-10 on our consolidated financial statements and we have not yet identified which transition method will be applied upon adoption.

In May 2016, the FASB issued ASU 2016-12, “Revenue from Contracts with Customers (Topic 606): Narrow-Scope Improvements and Practical Expedients.” ASU 2016-12 clarifies ASU 2014-09 “Revenue from Contracts with Customers (Topic 606)” guidance on (i) assessing collectability, (ii) presenting sales tax, (iii) measuring non-cash consideration and (iv) certain transition matters. ASU 2016-12 is effective concurrently with ASU 2014-09 which we are required to adopt in the first quarter of fiscal year 2018. Early adoption is permitted. The guidance permits companies to either apply the requirements retrospectively to all prior periods presented, or apply the requirements in the year of adoption, through cumulative adjustment. We are currently evaluating the potential impact of the pending adoption of ASU 2016-12 on our consolidated financial statements.

In June 2016, the FASB issued ASU 2016-13, “Financial Instruments - Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments.” ASU 2016-13 introduces an approach based on expected losses to estimate credit losses on certain types of financial instruments. ASU 2016-13 also modifies the impairment model for available-for-sale debt securities and provides for a simplified accounting model for purchased financial assets with credit deterioration since their origination. ASU 2016-13 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2019. Early adoption is permitted for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2018. The guidance requires companies to apply the requirements in the year of adoption through cumulative adjustment with some aspects of the update requiring a prospective transition approach. We are currently evaluating the potential impact of the pending adoption of ASU 2016-13 on our consolidated financial statements.

In August 2016, the FASB issued ASU 2016-15, “Statement of Cash Flows (Topic 230): Classification of Certain Cash Receipts and Cash Payments (a consensus of the Emerging Issues Task Force).” ASU 2016-15 is intended to reduce diversity in practice in how certain transactions are classified in the statement of cash flows. ASU 2016-15 addresses eight classification issues related to the statement of cash flows: (i) debt prepayment or debt extinguishment, (ii) settlement of zero-coupon bonds, (iii) contingent consideration payments made after a business combination, (iv) proceeds from the settlement of insurance claims, (v) proceeds from the settlement of corporate-owned life insurance policies, including bank-owned life insurance policies, (vi) distributions received from equity method invitees, (vii) beneficial interest in securitizations transactions, and (viii) separately identifiable cash flows and application of the predominance principle. ASU 2016-15 is effective for fiscal years, and interim periods within those fiscal years, beginning after December 15, 2017. Early adoption is permitted. The guidance requires companies to apply the requirements retrospectively to all prior periods presented. If it is impracticable for a company to apply ASU 2016-15 retrospectively, requirements may be applied prospectively as of the earliest date practicable. We are currently evaluating the potential impact of the pending adoption of ASU 2016-15 on our consolidated financial statements.