Summary of Significant Accounting Policies	12 Months Ended
Dec. 31, 2015
Accounting Policies [Abstract]
Summary of Significant Accounting Policies	SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Business. First Interstate BancSystem, Inc. (the “Parent Company” and collectively with its subsidiaries, the “Company”) is a financial and bank holding company that, through the branch offices of its bank subsidiary, provides a comprehensive range of banking products and services to individuals, businesses, municipalities and other entities throughout Montana, Wyoming and South Dakota. In addition to its primary emphasis on commercial and consumer banking services, the Company also offers trust, employee benefit, investment and insurance services through its bank subsidiary. The Company is subject to competition from other financial institutions and nonbank financial companies, and is also subject to the regulations of various government agencies and undergoes periodic examinations by those regulatory authorities. Basis of Presentation. The Company’s consolidated financial statements include the accounts of the Parent Company and its operating subsidiaries. As of December 31, 2015, the Company had one significant subsidiary, First Interstate Bank (“FIB”). All significant intercompany balances and transactions have been eliminated in consolidation. Certain reclassifications, none of which were material, have been made in the consolidated financial statements for 2014 and 2013 to conform to the 2015 presentation. These reclassifications did not change previously reported net income or stockholders’ equity. Equity Method Investments. The Company has investments in real estate joint ventures that are not consolidated because the Company does not own a majority voting interest, control the operations or receive a majority of the losses or earnings of the joint venture. These joint ventures are accounted for using the equity method of accounting whereby the Company initially records its investment at cost (or fair value at the date of acquisition) and then subsequently adjusts the carrying value for the Company’s proportionate share of distributions and earnings or losses of the joint ventures. Variable Interest Entities. The Company’s wholly-owned business trusts, FI Statutory Trust I (“Trust I”), FI Capital Trust II (“Trust II”), FI Statutory Trust III (“Trust III”), FI Capital Trust IV (“Trust IV”), FI Statutory Trust V (“Trust V”) and FI Statutory Trust VI (“Trust VI”) are variable interest entities for which the Company is not a primary beneficiary. Accordingly, the accounts of Trust I, Trust II, Trust III, Trust IV, Trust V and Trust VI are not included in the accompanying consolidated financial statements, and are instead accounted for using the equity method of accounting. The Company has equity investments in variable interest Certified Development Entities (“CDEs”) which have received allocations under the New Markets Tax Credits Program. The underlying activities of the CDEs are community development projects designed primarily to promote community welfare, such as economic rehabilitation and development of low-income areas by providing housing, services, or jobs for residents. The maximum exposure to loss in the CDEs is the amount of equity invested and credit extended by the Company. The Company has credit protection in the form of indemnification agreements, guarantees, and collateral arrangements. As the primary beneficiary of these variable interest entities, the Company’s consolidated financial statements include the assets, liabilities, and results of operations of the CDEs. The primary activities of the CDEs are recognized in interest and fees on loans, other non-interest income and long-term debt interest expense on the Company’s statements of operations. Related cash flows are recognized in loans originated, principal collected on loans and advances or repayments of long-term debt. Assets Held in Fiduciary or Agency Capacity. The Company holds certain trust assets in a fiduciary or agency capacity. The Company also purchases and sells federal funds as an agent. These and other assets held in an agency or fiduciary capacity are not assets of the Company and, accordingly, are not included in the accompanying consolidated financial statements. Use of Estimates. The preparation of consolidated financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements and income and expenses during the reporting period. Actual results could differ from those estimates. Material estimates that are particularly susceptible to change relate to the determination of the allowance for loan losses, the valuation of goodwill, fair valuations of investment securities and other financial instruments and the status of loss contingencies. Cash and Cash Equivalents. For purposes of reporting cash flows, cash and cash equivalents include cash on hand, amounts due from banks, federal funds sold for one day periods and interest bearing deposits in banks with original maturities of less than three months. As of December 31, 2015 and 2014, the Company had cash of $636,345 and $631,562, respectively, on deposit with the Federal Reserve Bank. In addition, the Company maintained compensating balances with the Federal Reserve Bank of approximately $10,031 and $7,507 as of December 31, 2015 and 2014, respectively, to reduce service charges for check clearing services. Investment Securities. Investments in debt securities that the Company has the positive intent and ability to hold to maturity are classified as held-to-maturity and carried at amortized cost. Investments in debt securities that may be sold in response to or in anticipation of changes in interest rates and resulting prepayment risk, or other factors, and marketable equity securities are classified as available-for-sale and carried at fair value. The unrealized gains and losses on these securities are reported, net of applicable income taxes, as a separate component of stockholders’ equity and comprehensive income. Management determines the appropriate classification of securities at the time of purchase and at each reporting date management reassesses the appropriateness of the classification. The amortized cost of debt securities classified as held-to-maturity or available-for-sale is adjusted for accretion of discounts to maturity and amortization of premiums over the estimated average life of the security, or in the case of callable securities, through the first call date, using the effective yield method. Such amortization and accretion is included in interest income. Realized gains and losses are included in investment securities gains (losses). Declines in the fair value of securities below their cost that are judged to be other-than-temporary are included in other expenses if the decline is related to credit losses. Other-than-temporary impairment losses related to other factors are recognized in other comprehensive income, net of income taxes. In estimating other-than-temporary impairment losses, the Company considers, among other things, the length of time and the extent to which the fair value has been less than cost, the financial condition and near-term prospects of the issuer and the intent and ability of the Company to retain its investment in the issuer for a period of time sufficient to allow for any anticipated recovery in fair value. The cost of securities sold is based on the specific identification method. Loans. Loans are reported at the principal amount outstanding. Interest income on loans is calculated using the simple interest method on the daily balance of the principal amount outstanding. Loan origination fees and certain direct origination costs are deferred, and the net amount is amortized as an adjustment of the related loan’s yield using a level yield method over the expected lives of the related loans. The accrual of interest on loans is discontinued when, in management’s opinion, the borrower may be unable to meet payment obligations as they become due or when a loan becomes contractually past due ninety days or more with respect to interest or principal, unless such past due loan is well secured and in the process of collection. When interest accrual is discontinued, all unpaid accrued interest is reversed against current period interest income. Interest income is subsequently recognized only to the extent cash payments are received in excess of principal due. Loans are returned to accrual status when all principal and interest amounts contractually due are brought current and when, in the opinion of management, the loans are estimated to be fully collectible as to both principal and interest. A loan is considered impaired when, based upon current information and events, it is probable that the Company will be unable to collect, on a timely basis, all amounts due according to the contractual terms of the loan’s original agreement. The amount of the impairment is measured using cash flows discounted at the loan’s effective interest rate, except when it is determined that the primary source of repayment for the loan is the operation or liquidation of the underlying collateral. In such cases, the current fair value of the collateral, reduced by anticipated selling costs, is used to measure impairment. The Company considers impaired loans to include all loans, except consumer loans, that are risk rated as doubtful or on which interest accrual has been discontinued or that have been renegotiated in a troubled debt restructuring. Interest payments received on impaired loans are applied based on whether they are on accrual or non-accrual status. Interest income recognized by the Company on impaired loans primarily relates to loans modified in troubled debt restructurings that remain on accrual status. Interest payments received on non-accrual impaired loans are applied to principal. Interest income is subsequently recognized only to the extent cash payments are received in excess of principal due. Loans acquired through the completion of a transfer, including loans acquired in business combinations, that have evidence of deterioration of credit quality since origination and for which it is probable, at acquisition, that the Company will be unable to collect all contractually required payments receivable are initially recorded at fair value (as determined by the present value of expected future cash flows) with no valuation allowance. The difference between the undiscounted cash flows expected at acquisition and the recorded fair value of the loan, or the “accretable yield,” is recognized as interest income on a level-yield method over the life of the loan. Contractually required payments for interest and principal that exceed the undiscounted cash flows expected at acquisition, or the “nonaccretable difference,” are not recognized as a yield adjustment, a loss accrual or a valuation allowance. Increases in expected cash flows subsequent to the initial investment are recognized prospectively through adjustment of the yield on the loan over its remaining life. Decreases in expected cash flows are recognized as impairment. Valuation allowances on these impaired loans reflect only losses incurred after the acquisition. A loan is considered a troubled debt restructuring when a borrower is experiencing financial difficulties that leads to a restructuring of the loan and the Company grants concessions to the borrower in the restructuring that it would not otherwise consider. These concessions may include rate reductions, principal forgiveness, extension of maturity date and other actions to minimize potential losses. Certain troubled debt restructurings are on non-accrual status at the time of restructuring and are returned to accrual status only after considering the borrower's sustained repayment performance in accordance with the restructuring agreement for a reasonable period of at least six months and management is reasonably assured of future performance. If the troubled debt restructuring meets these performance criteria and the interest rate granted at the modification is equal to or greater than the rate that the Company was willing to accept at the time of the restructuring for a new loan with comparable risk, then the loan will no longer be disclosed as a troubled debt restructuring although they continue to be individually evaluated for impairment and disclosed as impaired loans. Included in loans are certain residential mortgage loans originated for sale. These loans are carried at the lower of aggregate cost or estimated market value. Market value is estimated based on binding contracts or quotes or bids from third party investors. Residential mortgages held for sale were $52,875 and $40,828 as of December 31, 2015 and 2014, respectively. Gains and losses on sales of mortgage loans are determined using the specific identification method and are included in income from the origination and sale of loans. As of December 31, 2015, the Company had no recorded investments in consumer mortgage loans secured by residential real estate for which formal foreclosure proceedings were in process. Allowance for Loan Losses. The allowance for loan losses is established through a provision for loan losses which is charged to expense. Loans, or portions thereof, are charged against the allowance for loan losses when management believes that the collectibility of the principal is unlikely or, with respect to consumer installment and credit card loans, according to established delinquency schedules. The allowance balance is an amount that management believes will be adequate to absorb known and inherent losses in the loan portfolio based upon quarterly analyses of the current risk characteristics of the loan portfolio, an assessment of individual problem loans and actual loss experience, industry concentrations and current economic factors and the estimated impact of current economic and environmental conditions on historical loss rates. Loans acquired in business combinations are recorded at their estimated fair values on the date of acquisition. Accordingly, no allowance for loan losses related to these loans is recorded at the date of transfer. An allowance for loan losses is recorded for credit deterioration occurring subsequent to the transfer date, if any. Goodwill. The excess purchase price over the fair value of net assets from acquisitions, or goodwill, is evaluated for impairment at least annually and on an interim basis if an event or circumstance indicates that it is likely impairment has occurred. Goodwill impairment is determined by comparing the fair value of a reporting unit to its carrying amount. In any given year the Company may elect to perform a qualitative assessment to determine whether it is more likely than not that the fair value of a reporting unit is in excess of its carrying value. If it is not more likely than not that the fair value of the reporting unit is in excess of the carrying value, or if the Company elects to bypass the qualitative assessment, a two-step quantitative impairment test is performed. In performing a quantitative test for impairment, the fair value of net assets is estimated based on analyses of the Company's market value, discounted cash flows and peer values. The determination of goodwill impairment is sensitive to market-based economics and other key assumptions used in determining or allocating fair value. Variability in the market and changes in assumptions or subjective measurements used to allocate fair value are reasonably possible and may have a material impact on our consolidated financial statements or results of operations. Core Deposit Intangibles. Core deposit intangibles represent the intangible value of depositor relationships resulting from deposit liabilities assumed and are amortized using an accelerated method based on the estimated weighted average useful lives of the related deposits. Accumulated core deposit intangibles amortization was $28,040 as of December 31, 2015 and $24,652 as of December 31, 2014. Amortization expense related to core deposit intangibles recorded as of December 31, 2015 is expected to total $3,220, $1,933, $1,414, $1,201, $988, and $1,833 in 2016, 2017, 2018, 2019, 2020, and thereafter, respectively. Mortgage Servicing Rights. The Company recognizes the rights to service mortgage loans for others, whether acquired or internally originated. Mortgage servicing rights are initially recorded at fair value based on comparable market data and are amortized in proportion to and over the period of estimated net servicing income. Mortgage servicing rights are evaluated quarterly for impairment by discounting the expected future cash flows, taking into consideration the estimated level of prepayments based on current industry expectations and the predominant risk characteristics of the underlying loans including loan type, note rate and loan term. Impairment adjustments, if any, are recorded through a valuation allowance. Premises and Equipment. Buildings, furniture and equipment are stated at cost less accumulated depreciation. Depreciation expense is computed using straight-line methods over estimated useful lives of 5 to 45 years for buildings and improvements and 4 to 15 years for furniture and equipment. Leasehold improvements and assets acquired under capital lease are amortized over the shorter of their estimated useful lives or the terms of the related leases. Land is recorded at cost. Company-Owned Life Insurance. Key executive and group life insurance policies are recorded at their cash surrender value. Separate account group life insurance policies are subject to a stable value contract that offsets the impact of interest rate fluctuations on the market value of the policies and are recorded at the stabilized investment value. Increases in the cash surrender or stabilized investment value of insurance policies, as well as insurance proceeds received, are recorded as other non-interest income, and are not subject to income taxes. Deferred Compensation Plan. The Company has a deferred compensation plan for the benefit of certain highly compensated officers and directors of the Company. The plan allows for discretionary employer contributions in excess of tax limits applicable to the Company's 401(k) and profit sharing plans and the deferral of salary, short-term incentives or director fees subject to certain limitations. Deferred compensation plan assets and liabilities are included in the Company's consolidated balance sheets at fair value. Deferred compensation plan income or expense, consisting solely of net realized and unrealized holding gains and losses on deferred compensation plan assets, is recorded as other income in the Company’s consolidated statements of income. The corresponding net realized and unrealized holding gains and losses on deferred compensation plan liabilities is offset against employee benefits expense. As of December 31, 2015 and 2014, deferred compensation plan assets were $10,149 and $10,312, respectively. Corresponding deferred compensation plan liabilities were $10,149 and $10,136 as of December 31, 2015 and 2014, respectively. During 2015, net realized and unrealized holding losses on deferred compensation plan assets of $585 were included in other non-interest income and employee benefits expense in the Company's consolidated statements of income. During 2014, net realized and unrealized holding gains on deferred compensation plan assets of $434 were included in other non-interest income and employee benefits expense in the Company's consolidated statements of income. Impairment of Long-Lived Assets. Long-lived assets, including premises and equipment and certain identifiable intangibles, are reviewed for impairment whenever events or changes in circumstances indicate the carrying amount of an asset may not be recoverable. The amount of the impairment loss, if any, is based on the asset’s fair value. Impairment losses of $806, $102 and $616 were recognized in other non-interest expense in 2015, 2014 and 2013, respectively. Other Real Estate Owned. Real estate acquired in satisfaction of loans is initially carried at current fair value less estimated selling costs. Any excess of loan carrying value over the fair value of the real estate acquired is recorded as a charge to the allowance for loan losses. Subsequent declines in fair value less estimated selling costs are included in OREO expense. Subsequent increases in fair value less estimated selling costs are recorded as a reduction in OREO expense to the extent of recognized losses. Operating expenses, net of related income, and gains or losses on sales are included in OREO expense. Write-downs of $207, $224 and $3,512 were recorded in 2015, 2014 and 2013, respectively. The carrying value of foreclosed residential real estate properties included in other real estate owned was $1,686 as of December 31, 2015, and $2,644 as of December 31, 2014. Restricted Equity Securities. The Company, as a member of the Federal Reserve Bank and the Federal Home Loan Bank (“FHLB”), is required to maintain investments in each of the organization’s capital stock. As of December 31, 2015, restricted equity securities of the Federal Reserve Bank and the Federal Home Loan Bank of $16,421 and $10,135, respectively, were included in other assets at cost. As of December 31, 2014, restricted equity securities of the Federal Reserve Bank and the Federal Home Loan Bank were $16,187 and $10,662, respectively. No ready market exists for these restricted equity securities, and they have no quoted market values. Restricted equity securities are periodically reviewed for impairment based on ultimate recovery of par value. The determination of whether a decline affects the ultimate recovery of par value is influenced by the significance of the decline compared to the cost basis of the restricted equity securities, the length of time a decline has persisted, the impact of legislative and regulatory changes on the issuing organizations and the liquidity positions of the issuing organizations. Based on management’s assessment, no impairment losses were recorded on restricted equity securities during 2015, 2014 or 2013. Derivatives and Hedging Activities. For asset and liability management purposes, the Company has entered into interest rate swap contracts to hedge against changes in forecasted cash flows due to interest rate exposures. Interest rate swaps are contracts in which a series of interest payments are exchanged over a prescribed period. The notional amount upon which the interest payments are based is not exchanged. The swap agreements are derivative instruments and convert a portion of the Company’s forecasted variable rate debt to a fixed rate (i.e., cash flow hedge) over the payment term of the interest rate swap. The effective portion of the gain or loss on cash flow hedging instruments is initially reported as a component of other comprehensive income and subsequently reclassified into earnings in the same period during which the transaction affects earnings. The ineffective portion of the gain or loss on derivative instruments, if any, is recognized in earnings. The Company does not enter into interest rate swap agreements for trading or speculative purposes. The Company also enters into certain interest rate swap contracts that are not designated as hedging instruments. These derivative contracts relate to transactions in which the Company enters into an interest rate swap with a customer while at the same time entering into an offsetting interest rate swap with a third party financial institution. Because the Company acts as an intermediary for the customer, changes in the fair value of the underlying derivative contracts for the most part offset each other and do not significantly impact the Company's results of operations. The Company has residential real estate derivatives for commitments to fund certain residential real estate loans (interest rate locks) to be sold into the secondary market and forward commitments for the future delivery of residential real estate loans to third party investors on a mandatory or best efforts basis. It is the Company’s practice to enter into forward commitments for the future delivery of residential real estate loans when interest rate lock commitments are entered into in order to economically hedge the effect of changes in interest rates resulting from its commitments to fund the loans. Income from Fiduciary Activities. Consistent with industry practice, income for trust services is recognized on the basis of cash received. However, use of this method in lieu of accrual basis accounting does not materially affect reported earnings. Earnings Per Common Share. Basic and diluted earnings per common share are calculated using a two-class method. Under the two-class method, basic earnings per common share is calculated by dividing net income available to common shareholders by the weighted average number of common shares outstanding during the period, excluding outstanding participating securities. Participating securities include non-vested performance restricted stock awards granted prior to 2014 and all non-vested time restricted stock awards. Diluted earnings per common share is calculated by dividing net income available to common shareholders by the weighted average number of common shares outstanding determined for the basic earnings per share calculation plus the dilutive effect of stock compensation using the treasury stock method. Income Taxes. The Parent Company and its subsidiaries have elected to be included in a consolidated federal income tax return. For state income tax purposes, the combined taxable income of the Parent Company and its subsidiaries is apportioned among the states in which operations take place. Federal and state income taxes attributable to the subsidiaries, computed on a separate return basis, are paid to or received from the Parent Company. The Company accounts for income taxes using the liability method. Under the liability method, deferred tax assets and liabilities are determined based on enacted income tax rates which will be in effect when the differences between the financial statement carrying values and tax bases of existing assets and liabilities are expected to be reported in taxable income. Positions taken in the Company’s tax returns may be subject to challenge by the taxing authorities upon examination. Uncertain tax positions are initially recognized in the financial statements when it is more likely than not the position will be sustained upon examination by the tax authorities. Such tax positions are both initially and subsequently measured as the largest amount of tax benefit that is greater than 50% likely of being realized upon settlement with the tax authority, assuming full knowledge of the position and all relevant facts. The Company provides for interest and, in some cases, penalties on tax positions that may be challenged by the taxing authorities. Interest expense is recognized beginning in the first period that such interest would begin accruing. Penalties are recognized in the period that the Company claims the position in the tax return. Interest and penalties on income tax uncertainties are classified within income tax expense in the income statement. With few exceptions, the Company is no longer subject to U.S. federal and state examinations by tax authorities for years before 2012. The Company had accrued interest of $235, $206 and $176 as of December 31, 2015, 2014 or 2013. The Company had no penalties as of December 31, 2015, 2014 or 2013. Comprehensive Income. Comprehensive income includes net income, as well as other changes in stockholders’ equity that result from transactions and economic events other than those with shareholders. In addition to net income, the Company’s comprehensive income includes the after tax effect of changes in unrealized gains and losses on available-for-sale investment securities and derivatives designated as cash flow hedges, changes in the unamortized gain or loss on available-for-sale investment securities transferred to held-to-maturity and changes in net actuarial gains and losses on defined benefit post-retirement benefits plans. Segment Reporting. An operating segment is defined as a component of a business for which separate financial information is available that is evaluated regularly by the chief operating decision maker in deciding how to allocate resources and evaluate performance. The Company has one operating segment, community banking, which encompasses commercial and consumer banking services offered to individuals, businesses, municipalities and other entities. Advertising Costs. Advertising costs are expensed as incurred. Advertising expense was $3,452, $3,734, and $3,532 in 2015, 2014 and 2013, respectively. Transfers of Financial Assets. Transfers of financial assets are accounted for as sales when control over the assets has been surrendered. Control over transferred assets is deemed to be surrendered when the assets have been isolated from the Company; the transferee obtains the right, free of conditions that constrain it from taking advantage of that right, to pledge or exchange the transferred assets; and, the Company does not maintain effective control over the transferred assets through an agreement to repurchase them before their maturity. Stock-Based Compensation. Compensation cost for all stock-based awards is measured at fair value on the date of grant and is recognized over the requisite service period for awards expected to vest. Stock-based compensation expense of $3,959, $3,634 and $2,936 for the years ended December 31, 2015, 2014 and 2013, respectively, is included in benefits expense in the Company’s consolidated statements of income. Related income tax benefits recognized for the years ended December 31, 2015, 2014 and 2013 were $1,443, $1,390 and $1,122, respectively. Fair Value Measurements. In general, fair value measurements are based upon quoted market prices, where available. If quoted market prices are not available, fair value measurements are estimated using relevant market information and other assumptions. Fair value estimates involve uncertainties and require some degree of judgment regarding interest rates, credit risk, prepayments and other factors. The use of different assumptions or estimation techniques may have a significant effect on the fair value amounts reported.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Business. First Interstate BancSystem, Inc. (the “Parent Company” and collectively with its subsidiaries, the “Company”) is a financial and bank holding company that, through the branch offices of its bank subsidiary, provides a comprehensive range of banking products and services to individuals, businesses, municipalities and other entities throughout Montana, Wyoming and South Dakota. In addition to its primary emphasis on commercial and consumer banking services, the Company also offers trust, employee benefit, investment and insurance services through its bank subsidiary. The Company is subject to competition from other financial institutions and nonbank financial companies, and is also subject to the regulations of various government agencies and undergoes periodic examinations by those regulatory authorities.

Basis of Presentation. The Company’s consolidated financial statements include the accounts of the Parent Company and its operating subsidiaries. As of December 31, 2015, the Company had one significant subsidiary, First Interstate Bank (“FIB”). All significant intercompany balances and transactions have been eliminated in consolidation. Certain reclassifications, none of which were material, have been made in the consolidated financial statements for 2014 and 2013 to conform to the 2015 presentation. These reclassifications did not change previously reported net income or stockholders’ equity.

Equity Method Investments. The Company has investments in real estate joint ventures that are not consolidated because the Company does not own a majority voting interest, control the operations or receive a majority of the losses or earnings of the joint venture. These joint ventures are accounted for using the equity method of accounting whereby the Company initially records its investment at cost (or fair value at the date of acquisition) and then subsequently adjusts the carrying value for the Company’s proportionate share of distributions and earnings or losses of the joint ventures.

Variable Interest Entities. The Company’s wholly-owned business trusts, FI Statutory Trust I (“Trust I”), FI Capital Trust II (“Trust II”), FI Statutory Trust III (“Trust III”), FI Capital Trust IV (“Trust IV”), FI Statutory Trust V (“Trust V”) and FI Statutory Trust VI (“Trust VI”) are variable interest entities for which the Company is not a primary beneficiary. Accordingly, the accounts of Trust I, Trust II, Trust III, Trust IV, Trust V and Trust VI are not included in the accompanying consolidated financial statements, and are instead accounted for using the equity method of accounting.

The Company has equity investments in variable interest Certified Development Entities (“CDEs”) which have received allocations under the New Markets Tax Credits Program. The underlying activities of the CDEs are community development projects designed primarily to promote community welfare, such as economic rehabilitation and development of low-income areas by providing housing, services, or jobs for residents. The maximum exposure to loss in the CDEs is the amount of equity invested and credit extended by the Company. The Company has credit protection in the form of indemnification agreements, guarantees, and collateral arrangements. As the primary beneficiary of these variable interest entities, the Company’s consolidated financial statements include the assets, liabilities, and results of operations of the CDEs. The primary activities of the CDEs are recognized in interest and fees on loans, other non-interest income and long-term debt interest expense on the Company’s statements of operations. Related cash flows are recognized in loans originated, principal collected on loans and advances or repayments of long-term debt.

Assets Held in Fiduciary or Agency Capacity. The Company holds certain trust assets in a fiduciary or agency capacity. The Company also purchases and sells federal funds as an agent. These and other assets held in an agency or fiduciary capacity are not assets of the Company and, accordingly, are not included in the accompanying consolidated financial statements.

Use of Estimates. The preparation of consolidated financial statements in conformity with accounting principles generally accepted in the United States of America requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent assets and liabilities at the date of the financial statements and income and expenses during the reporting period. Actual results could differ from those estimates. Material estimates that are particularly susceptible to change relate to the determination of the allowance for loan losses, the valuation of goodwill, fair valuations of investment securities and other financial instruments and the status of loss contingencies.

Cash and Cash Equivalents. For purposes of reporting cash flows, cash and cash equivalents include cash on hand, amounts due from banks, federal funds sold for one day periods and interest bearing deposits in banks with original maturities of less than three months. As of December 31, 2015 and 2014, the Company had cash of $636,345 and $631,562, respectively, on deposit with the Federal Reserve Bank. In addition, the Company maintained compensating balances with the Federal Reserve Bank of approximately $10,031 and $7,507 as of December 31, 2015 and 2014, respectively, to reduce service charges for check clearing services.

Investment Securities. Investments in debt securities that the Company has the positive intent and ability to hold to maturity are classified as held-to-maturity and carried at amortized cost. Investments in debt securities that may be sold in response to or in anticipation of changes in interest rates and resulting prepayment risk, or other factors, and marketable equity securities are classified as available-for-sale and carried at fair value. The unrealized gains and losses on these securities are reported, net of applicable income taxes, as a separate component of stockholders’ equity and comprehensive income. Management determines the appropriate classification of securities at the time of purchase and at each reporting date management reassesses the appropriateness of the classification.

The amortized cost of debt securities classified as held-to-maturity or available-for-sale is adjusted for accretion of discounts to maturity and amortization of premiums over the estimated average life of the security, or in the case of callable securities, through the first call date, using the effective yield method. Such amortization and accretion is included in interest income. Realized gains and losses are included in investment securities gains (losses). Declines in the fair value of securities below their cost that are judged to be other-than-temporary are included in other expenses if the decline is related to credit losses. Other-than-temporary impairment losses related to other factors are recognized in other comprehensive income, net of income taxes. In estimating other-than-temporary impairment losses, the Company considers, among other things, the length of time and the extent to which the fair value has been less than cost, the financial condition and near-term prospects of the issuer and the intent and ability of the Company to retain its investment in the issuer for a period of time sufficient to allow for any anticipated recovery in fair value. The cost of securities sold is based on the specific identification method.

Loans. Loans are reported at the principal amount outstanding. Interest income on loans is calculated using the simple interest method on the daily balance of the principal amount outstanding. Loan origination fees and certain direct origination costs are deferred, and the net amount is amortized as an adjustment of the related loan’s yield using a level yield method over the expected lives of the related loans.

The accrual of interest on loans is discontinued when, in management’s opinion, the borrower may be unable to meet payment obligations as they become due or when a loan becomes contractually past due ninety days or more with respect to interest or principal, unless such past due loan is well secured and in the process of collection. When interest accrual is discontinued, all unpaid accrued interest is reversed against current period interest income. Interest income is subsequently recognized only to the extent cash payments are received in excess of principal due. Loans are returned to accrual status when all principal and interest amounts contractually due are brought current and when, in the opinion of management, the loans are estimated to be fully collectible as to both principal and interest.

A loan is considered impaired when, based upon current information and events, it is probable that the Company will be unable to collect, on a timely basis, all amounts due according to the contractual terms of the loan’s original agreement. The amount of the impairment is measured using cash flows discounted at the loan’s effective interest rate, except when it is determined that the primary source of repayment for the loan is the operation or liquidation of the underlying collateral. In such cases, the current fair value of the collateral, reduced by anticipated selling costs, is used to measure impairment. The Company considers impaired loans to include all loans, except consumer loans, that are risk rated as doubtful or on which interest accrual has been discontinued or that have been renegotiated in a troubled debt restructuring. Interest payments received on impaired loans are applied based on whether they are on accrual or non-accrual status. Interest income recognized by the Company on impaired loans primarily relates to loans modified in troubled debt restructurings that remain on accrual status. Interest payments received on non-accrual impaired loans are applied to principal. Interest income is subsequently recognized only to the extent cash payments are received in excess of principal due.

Loans acquired through the completion of a transfer, including loans acquired in business combinations, that have evidence of deterioration of credit quality since origination and for which it is probable, at acquisition, that the Company will be unable to collect all contractually required payments receivable are initially recorded at fair value (as determined by the present value of expected future cash flows) with no valuation allowance. The difference between the undiscounted cash flows expected at acquisition and the recorded fair value of the loan, or the “accretable yield,” is recognized as interest income on a level-yield method over the life of the loan. Contractually required payments for interest and principal that exceed the undiscounted cash flows expected at acquisition, or the “nonaccretable difference,” are not recognized as a yield adjustment, a loss accrual or a valuation allowance. Increases in expected cash flows subsequent to the initial investment are recognized prospectively through adjustment of the yield on the loan over its remaining life. Decreases in expected cash flows are recognized as impairment. Valuation allowances on these impaired loans reflect only losses incurred after the acquisition.

A loan is considered a troubled debt restructuring when a borrower is experiencing financial difficulties that leads to a restructuring of the loan and the Company grants concessions to the borrower in the restructuring that it would not otherwise consider. These concessions may include rate reductions, principal forgiveness, extension of maturity date and other actions to minimize potential losses. Certain troubled debt restructurings are on non-accrual status at the time of restructuring and are returned to accrual status only after considering the borrower's sustained repayment performance in accordance with the restructuring agreement for a reasonable period of at least six months and management is reasonably assured of future performance. If the troubled debt restructuring meets these performance criteria and the interest rate granted at the modification is equal to or greater than the rate that the Company was willing to accept at the time of the restructuring for a new loan with comparable risk, then the loan will no longer be disclosed as a troubled debt restructuring although they continue to be individually evaluated for impairment and disclosed as impaired loans.

Included in loans are certain residential mortgage loans originated for sale. These loans are carried at the lower of aggregate cost or estimated market value. Market value is estimated based on binding contracts or quotes or bids from third party investors. Residential mortgages held for sale were $52,875 and $40,828 as of December 31, 2015 and 2014, respectively. Gains and losses on sales of mortgage loans are determined using the specific identification method and are included in income from the origination and sale of loans.

As of December 31, 2015, the Company had no recorded investments in consumer mortgage loans secured by residential real estate for which formal foreclosure proceedings were in process.

Allowance for Loan Losses. The allowance for loan losses is established through a provision for loan losses which is charged to expense. Loans, or portions thereof, are charged against the allowance for loan losses when management believes that the collectibility of the principal is unlikely or, with respect to consumer installment and credit card loans, according to established delinquency schedules. The allowance balance is an amount that management believes will be adequate to absorb known and inherent losses in the loan portfolio based upon quarterly analyses of the current risk characteristics of the loan portfolio, an assessment of individual problem loans and actual loss experience, industry concentrations and current economic factors and the estimated impact of current economic and environmental conditions on historical loss rates.

Loans acquired in business combinations are recorded at their estimated fair values on the date of acquisition. Accordingly, no allowance for loan losses related to these loans is recorded at the date of transfer. An allowance for loan losses is recorded for credit deterioration occurring subsequent to the transfer date, if any.

Goodwill. The excess purchase price over the fair value of net assets from acquisitions, or goodwill, is evaluated for impairment at least annually and on an interim basis if an event or circumstance indicates that it is likely impairment has occurred. Goodwill impairment is determined by comparing the fair value of a reporting unit to its carrying amount. In any given year the Company may elect to perform a qualitative assessment to determine whether it is more likely than not that the fair value of a reporting unit is in excess of its carrying value. If it is not more likely than not that the fair value of the reporting unit is in excess of the carrying value, or if the Company elects to bypass the qualitative assessment, a two-step quantitative impairment test is performed. In performing a quantitative test for impairment, the fair value of net assets is estimated based on analyses of the Company's market value, discounted cash flows and peer values. The determination of goodwill impairment is sensitive to market-based economics and other key assumptions used in determining or allocating fair value. Variability in the market and changes in assumptions or subjective measurements used to allocate fair value are reasonably possible and may have a material impact on our consolidated financial statements or results of operations.

Core Deposit Intangibles. Core deposit intangibles represent the intangible value of depositor relationships resulting from deposit liabilities assumed and are amortized using an accelerated method based on the estimated weighted average useful lives of the related deposits. Accumulated core deposit intangibles amortization was $28,040 as of December 31, 2015 and $24,652 as of December 31, 2014. Amortization expense related to core deposit intangibles recorded as of December 31, 2015 is expected to total $3,220, $1,933, $1,414, $1,201, $988, and $1,833 in 2016, 2017, 2018, 2019, 2020, and thereafter, respectively.

Mortgage Servicing Rights. The Company recognizes the rights to service mortgage loans for others, whether acquired or internally originated. Mortgage servicing rights are initially recorded at fair value based on comparable market data and are amortized in proportion to and over the period of estimated net servicing income. Mortgage servicing rights are evaluated quarterly for impairment by discounting the expected future cash flows, taking into consideration the estimated level of prepayments based on current industry expectations and the predominant risk characteristics of the underlying loans including loan type, note rate and loan term. Impairment adjustments, if any, are recorded through a valuation allowance.

Premises and Equipment. Buildings, furniture and equipment are stated at cost less accumulated depreciation. Depreciation expense is computed using straight-line methods over estimated useful lives of 5 to 45 years for buildings and improvements and 4 to 15 years for furniture and equipment. Leasehold improvements and assets acquired under capital lease are amortized over the shorter of their estimated useful lives or the terms of the related leases. Land is recorded at cost.

Company-Owned Life Insurance. Key executive and group life insurance policies are recorded at their cash surrender value. Separate account group life insurance policies are subject to a stable value contract that offsets the impact of interest rate fluctuations on the market value of the policies and are recorded at the stabilized investment value. Increases in the cash surrender or stabilized investment value of insurance policies, as well as insurance proceeds received, are recorded as other non-interest income, and are not subject to income taxes.

Deferred Compensation Plan. The Company has a deferred compensation plan for the benefit of certain highly compensated officers and directors of the Company. The plan allows for discretionary employer contributions in excess of tax limits applicable to the Company's 401(k) and profit sharing plans and the deferral of salary, short-term incentives or director fees subject to certain limitations. Deferred compensation plan assets and liabilities are included in the Company's consolidated balance sheets at fair value. Deferred compensation plan income or expense, consisting solely of net realized and unrealized holding gains and losses on deferred compensation plan assets, is recorded as other income in the Company’s consolidated statements of income. The corresponding net realized and unrealized holding gains and losses on deferred compensation plan liabilities is offset against employee benefits expense.

As of December 31, 2015 and 2014, deferred compensation plan assets were $10,149 and $10,312, respectively. Corresponding deferred compensation plan liabilities were $10,149 and $10,136 as of December 31, 2015 and 2014, respectively. During 2015, net realized and unrealized holding losses on deferred compensation plan assets of $585 were included in other non-interest income and employee benefits expense in the Company's consolidated statements of income. During 2014, net realized and unrealized holding gains on deferred compensation plan assets of $434 were included in other non-interest income and employee benefits expense in the Company's consolidated statements of income.

Impairment of Long-Lived Assets. Long-lived assets, including premises and equipment and certain identifiable intangibles, are reviewed for impairment whenever events or changes in circumstances indicate the carrying amount of an asset may not be recoverable. The amount of the impairment loss, if any, is based on the asset’s fair value. Impairment losses of $806, $102 and $616 were recognized in other non-interest expense in 2015, 2014 and 2013, respectively.

Other Real Estate Owned. Real estate acquired in satisfaction of loans is initially carried at current fair value less estimated selling costs. Any excess of loan carrying value over the fair value of the real estate acquired is recorded as a charge to the allowance for loan losses. Subsequent declines in fair value less estimated selling costs are included in OREO expense. Subsequent increases in fair value less estimated selling costs are recorded as a reduction in OREO expense to the extent of recognized losses. Operating expenses, net of related income, and gains or losses on sales are included in OREO expense. Write-downs of $207, $224 and $3,512 were recorded in 2015, 2014 and 2013, respectively. The carrying value of foreclosed residential real estate properties included in other real estate owned was $1,686 as of December 31, 2015, and $2,644 as of December 31, 2014.

Restricted Equity Securities. The Company, as a member of the Federal Reserve Bank and the Federal Home Loan Bank (“FHLB”), is required to maintain investments in each of the organization’s capital stock. As of December 31, 2015, restricted equity securities of the Federal Reserve Bank and the Federal Home Loan Bank of $16,421 and $10,135, respectively, were included in other assets at cost. As of December 31, 2014, restricted equity securities of the Federal Reserve Bank and the Federal Home Loan Bank were $16,187 and $10,662, respectively. No ready market exists for these restricted equity securities, and they have no quoted market values. Restricted equity securities are periodically reviewed for impairment based on ultimate recovery of par value. The determination of whether a decline affects the ultimate recovery of par value is influenced by the significance of the decline compared to the cost basis of the restricted equity securities, the length of time a decline has persisted, the impact of legislative and regulatory changes on the issuing organizations and the liquidity positions of the issuing organizations. Based on management’s assessment, no impairment losses were recorded on restricted equity securities during 2015, 2014 or 2013.

Derivatives and Hedging Activities. For asset and liability management purposes, the Company has entered into interest rate swap contracts to hedge against changes in forecasted cash flows due to interest rate exposures. Interest rate swaps are contracts in which a series of interest payments are exchanged over a prescribed period. The notional amount upon which the interest payments are based is not exchanged. The swap agreements are derivative instruments and convert a portion of the Company’s forecasted variable rate debt to a fixed rate (i.e., cash flow hedge) over the payment term of the interest rate swap. The effective portion of the gain or loss on cash flow hedging instruments is initially reported as a component of other comprehensive income and subsequently reclassified into earnings in the same period during which the transaction affects earnings. The ineffective portion of the gain or loss on derivative instruments, if any, is recognized in earnings. The Company does not enter into interest rate swap agreements for trading or speculative purposes.

The Company also enters into certain interest rate swap contracts that are not designated as hedging instruments. These derivative contracts relate to transactions in which the Company enters into an interest rate swap with a customer while at the same time entering into an offsetting interest rate swap with a third party financial institution. Because the Company acts as an intermediary for the customer, changes in the fair value of the underlying derivative contracts for the most part offset each other and do not significantly impact the Company's results of operations.

The Company has residential real estate derivatives for commitments to fund certain residential real estate loans (interest rate locks) to be sold into the secondary market and forward commitments for the future delivery of residential real estate loans to third party investors on a mandatory or best efforts basis. It is the Company’s practice to enter into forward commitments for the future delivery of residential real estate loans when interest rate lock commitments are entered into in order to economically hedge the effect of changes in interest rates resulting from its commitments to fund the loans.

Income from Fiduciary Activities. Consistent with industry practice, income for trust services is recognized on the basis of cash received. However, use of this method in lieu of accrual basis accounting does not materially affect reported earnings.

Earnings Per Common Share. Basic and diluted earnings per common share are calculated using a two-class method. Under the two-class method, basic earnings per common share is calculated by dividing net income available to common shareholders by the weighted average number of common shares outstanding during the period, excluding outstanding participating securities. Participating securities include non-vested performance restricted stock awards granted prior to 2014 and all non-vested time restricted stock awards. Diluted earnings per common share is calculated by dividing net income available to common shareholders by the weighted average number of common shares outstanding determined for the basic earnings per share calculation plus the dilutive effect of stock compensation using the treasury stock method.

Income Taxes. The Parent Company and its subsidiaries have elected to be included in a consolidated federal income tax return. For state income tax purposes, the combined taxable income of the Parent Company and its subsidiaries is apportioned among the states in which operations take place. Federal and state income taxes attributable to the subsidiaries, computed on a separate return basis, are paid to or received from the Parent Company.

The Company accounts for income taxes using the liability method. Under the liability method, deferred tax assets and liabilities are determined based on enacted income tax rates which will be in effect when the differences between the financial statement carrying values and tax bases of existing assets and liabilities are expected to be reported in taxable income.

Positions taken in the Company’s tax returns may be subject to challenge by the taxing authorities upon examination. Uncertain tax positions are initially recognized in the financial statements when it is more likely than not the position will be sustained upon examination by the tax authorities. Such tax positions are both initially and subsequently measured as the largest amount of tax benefit that is greater than 50% likely of being realized upon settlement with the tax authority, assuming full knowledge of the position and all relevant facts. The Company provides for interest and, in some cases, penalties on tax positions that may be challenged by the taxing authorities. Interest expense is recognized beginning in the first period that such interest would begin accruing. Penalties are recognized in the period that the Company claims the position in the tax return. Interest and penalties on income tax uncertainties are classified within income tax expense in the income statement. With few exceptions, the Company is no longer subject to U.S. federal and state examinations by tax authorities for years before 2012. The Company had accrued interest of $235, $206 and $176 as of December 31, 2015, 2014 or 2013. The Company had no penalties as of December 31, 2015, 2014 or 2013.

Comprehensive Income. Comprehensive income includes net income, as well as other changes in stockholders’ equity that result from transactions and economic events other than those with shareholders. In addition to net income, the Company’s comprehensive income includes the after tax effect of changes in unrealized gains and losses on available-for-sale investment securities and derivatives designated as cash flow hedges, changes in the unamortized gain or loss on available-for-sale investment securities transferred to held-to-maturity and changes in net actuarial gains and losses on defined benefit post-retirement benefits plans.

Segment Reporting. An operating segment is defined as a component of a business for which separate financial information is available that is evaluated regularly by the chief operating decision maker in deciding how to allocate resources and evaluate performance. The Company has one operating segment, community banking, which encompasses commercial and consumer banking services offered to individuals, businesses, municipalities and other entities.

Advertising Costs. Advertising costs are expensed as incurred. Advertising expense was $3,452, $3,734, and $3,532 in 2015, 2014 and 2013, respectively.

Transfers of Financial Assets. Transfers of financial assets are accounted for as sales when control over the assets has been surrendered. Control over transferred assets is deemed to be surrendered when the assets have been isolated from the Company; the transferee obtains the right, free of conditions that constrain it from taking advantage of that right, to pledge or exchange the transferred assets; and, the Company does not maintain effective control over the transferred assets through an agreement to repurchase them before their maturity.

Stock-Based Compensation. Compensation cost for all stock-based awards is measured at fair value on the date of grant and is recognized over the requisite service period for awards expected to vest. Stock-based compensation expense of $3,959, $3,634 and $2,936 for the years ended December 31, 2015, 2014 and 2013, respectively, is included in benefits expense in the Company’s consolidated statements of income. Related income tax benefits recognized for the years ended December 31, 2015, 2014 and 2013 were $1,443, $1,390 and $1,122, respectively.

Fair Value Measurements. In general, fair value measurements are based upon quoted market prices, where available. If quoted market prices are not available, fair value measurements are estimated using relevant market information and other assumptions. Fair value estimates involve uncertainties and require some degree of judgment regarding interest rates, credit risk, prepayments and other factors. The use of different assumptions or estimation techniques may have a significant effect on the fair value amounts reported.