Summary of Significant Accounting Policies (Policies)	12 Months Ended
Summary of Significant Accounting Policies (Policies)	Dec. 31, 2016
Accounting Policies [Abstract]
Principles of Consolidation	Principles of Consolidation The consolidated financial statements include the accounts of the Company and the Bank. All intercompany accounts and transactions have been eliminated in consolidation.
Subsequent Events	Subsequent Events Events occurring subsequent to December 31, 2016, have been evaluated as to their potential impact to the financial statements.
Use of Estimates in Preparation of Financial Statements	Use of Estimates in Preparation of Financial Statements In preparing the financial statements in accordance with accounting principles generally accepted in the United States of America ("GAAP"), Management is required to make estimates and assumptions that affect the reported amounts of assets and liabilities and disclosures of contingent assets and liabilities as of the date of the balance sheet and revenues and expenses for the reporting period. Actual results could differ significantly from those estimates. Material estimates that are particularly susceptible to significant change in the near-term relate to the determination of the allowance for loan losses, goodwill, the valuation of mortgage servicing rights, and other-than-temporary impairment of securities.
Investment Securities	Investment Securities Investment securities are classified as available for sale or held to maturity when purchased. There are no trading account securities. Securities available for sale consist primarily of debt securities which Management intends to hold for indefinite periods of time. They may be used as part of the Bank's funds management strategy, and may be sold in response to changes in interest rates or prepayment risk, changes in liquidity needs, or for other reasons. They are accounted for at fair value, with unrealized gains or losses adjusted through shareholders' equity, net of related income taxes. The cost basis is adjusted for the amortization of premiums and accretion of discounts. Securities to be held to maturity consist primarily of debt securities which Management has acquired solely for long-term investment purposes, rather than for purposes of trading or future sale. For securities to be held to maturity, Management has the intent and the Bank has the ability to hold such securities until their respective maturity dates. Such securities are carried at cost adjusted for the amortization of premiums and accretion of discounts. Investment securities transactions are accounted for on a settlement date basis; reported amounts would not be materially different from those accounted for on a trade date basis. Gains and losses on the sales of investment securities are determined using the amortized cost of the specifically identified security. For declines in the fair value of individual debt securities available for sale below their cost that are deemed to be other than temporary, where the Company does not intend to sell the security and it is more likely than not that the Company will not be required to sell the security before recovery of its amortized cost basis, the other-than-temporary decline in the fair value of the debt security related to 1) credit loss is recognized in earnings and 2) other factors is recognized in other comprehensive income or loss. Credit loss is deemed to exist if the present value of expected future cash flows using the effective rate at acquisition is less than the amortized cost basis of the debt security. For individual debt securities where the Company intends to sell the security or more likely than not will be required to sell the security before recovery of its amortized cost, the other-than-temporary impairment is recognized in earnings equal to the entire difference between the security's cost basis and its fair value at the balance sheet date.
Derivative Financial Instruments Designated as Hedges	Derivative Financial Instruments Designated as Hedges The Company recognizes all derivatives in the consolidated balance sheets at fair value. On the date the Company enters into the derivative contract, the Company designates the derivative as a hedge of either a forecasted transaction or the variability of cash flows to be received or paid related to a recognized asset or liability (“cash flow hedge”), a hedge of the fair value of a recognized asset or liability or of an unrecognized firm commitment (“fair value hedge”), or a held for trading instrument (“trading instrument”). The Company formally documents relationships between hedging instruments and hedged items, as well as its risk management objectives and strategy for undertaking various hedge transactions. The Company also assesses, both at the hedge’s inception and on an ongoing basis, whether the derivatives that are used in hedging transactions are effective in offsetting changes in cash flows or fair values of hedged items. Changes in fair value of a derivative that is effective and that qualifies as a cash flow hedge are recorded in other comprehensive income (loss) and are reclassified into earnings when the forecasted transaction or related cash flows affect earnings. Changes in fair value of a derivative that qualifies as a fair value hedge and the change in fair value of the hedged item are both recorded in earnings and offset each other when the transaction is effective. Those derivatives that are classified as trading instruments are recorded at fair value with changes in fair value recorded in earnings. The Company discontinues hedge accounting when it determines that the derivative is no longer effective in offsetting changes in the cash flows of the hedged item, that it is unlikely that the forecasted transaction will occur, or that the designation of the derivative as a hedging instrument is no longer appropriate.
Loans Held for Sale	Loans Held for Sale Loans held for sale consist of residential real estate mortgage loans and are carried at the lower of aggregate cost or fair value, as determined by current investor yield requirements.
Loans	Loans Loans are generally reported at their outstanding principal balances, adjusted for chargeoffs, the allowance for loan losses and any deferred fees or costs to originate loans. Loan commitments are recorded when funded.
Loan Fees and Costs	Loan Fees and Costs Loan origination fees and certain direct loan origination costs are deferred and recognized in interest income as an adjustment to the loan yield over the life of the related loans. The unamortized net deferred fees and costs are included on the balance sheets with the related loan balances, and the amortization is included with the related interest income.
Allowance for Loan Losses	Allowance for Loan Losses Loans considered to be uncollectible are charged against the allowance for loan losses. The allowance for loan losses is maintained at a level determined by Management to be appropriate to absorb probable losses. This allowance is increased by provisions charged to operating expenses and recoveries on loans previously charged off. Arriving at an appropriate level of allowance for loan losses necessarily involves a high degree of judgment. In determining the appropriate level of allowance for loan losses, Management takes into consideration several factors, including reviews of individual non-performing loans and performing loans listed on the watch report requiring periodic evaluation, loan portfolio size by category, recent loss experience, delinquency trends and current economic conditions. For all loan classes, loans over 30 days past due are considered delinquent. Impaired loans include restructured loans and loans placed on non-accrual status when, based on current information and events, it is probable that the Bank will be unable to collect all amounts due according to the contractual terms of the loan agreement. These loans are measured at the present value of expected future cash flows discounted at the loan's effective interest rate or at the fair value of the collateral if the loan is collateral dependent. Management takes into consideration impaired loans in addition to the above mentioned factors in determining the appropriate level of allowance for loan losses.
Troubled Debt Restructured	Troubled Debt Restructured A troubled debt restructured ("TDR") constitutes a restructuring of debt if the Bank, for economic or legal reasons related to the borrower's financial difficulties, grants a concession to the borrower that it would not otherwise consider. To determine whether or not a loan should be classified as a TDR, Management evaluates a loan to first determine if the borrower demonstrates financial difficulty. Common indicators of this include past due status with bank obligations, substandard credit bureau reports, or an inability to refinance with another lender. If the borrower is experiencing financial difficulty and concessions are granted, such as maturity date extension, interest rate adjustments to below market pricing, or a deferment of payments, the loan will generally be classified as a TDR.
Accrual of Interest Income and Expense	Accrual of Interest Income and Expense Interest on loans and investment securities is taken into income using methods which relate the income earned to the balances of loans and investment securities outstanding. Interest expense on liabilities is derived by applying applicable interest rates to principal amounts outstanding. For all classes of loans, recording of interest income on problem loans, which includes impaired loans, ceases when collectibility of principal and interest within a reasonable period of time becomes doubtful. Cash payments received on non-accrual loans, which includes impaired loans, are applied to reduce the loan's principal balance until the remaining principal balance is deemed collectible, after which interest is recognized when collected. As a general rule, a loan may be restored to accrual status when payments are current for a substantial period of time, generally six months, and repayment of the remaining contractual amounts is expected or when it otherwise becomes well secured and in the process of collection.
Premises and Equipment	Premises and Equipment Premises, furniture and equipment are stated at cost, less accumulated depreciation. Depreciation expense is computed by straight-line methods over the asset's estimated useful life.
Other Real Estate Owned (OREO)	Other Real Estate Owned ("OREO") Real estate acquired by foreclosure or deed in lieu of foreclosure is transferred to OREO and recorded at fair value, less estimated costs to sell, based on appraised value at the date actually or constructively received. Loan losses arising from the acquisition of such property are charged against the allowance for loan losses. Subsequent provisions to reduce the carrying value of a property are recorded to the allowance for OREO losses and a charge to operations on a specific property basis.
Goodwill and Identified Intangible Assets	Goodwill and Identified Intangible Assets Intangible assets include the excess of the purchase price over the fair value of net assets acquired (goodwill) from the acquisition of FNB Bankshares in 2005 as well as the core deposit intangible related to the same acquisition. The core deposit intangible is amortized on a straight-line basis over ten years. There was no annual amortization expense for 2016 as the expense is now fully amortized. For 2015 and 2014 the annual amortization expense was $15,000 and $283,000, respectfully. Intangible assets also include the goodwill and core deposit intangible from the 2012 acquisition of a bank branch in Rockland, Maine and a bank building in Bangor, Maine. The core deposit intangible will be amortized on a straight-line basis over ten years. Annual amortization expense for 2016, 2015 and 2014 was $43,000, and the amortization expense for each year until fully amortized will be $43,000. The straight-line basis is used because the Company does not expect significant run off in the core deposits acquired. The Company annually evaluates goodwill, and periodically evaluates other intangible assets, for impairment. At December 31, 2016, the Company determined goodwill and other intangible assets were not impaired.
Income Taxes	Income Taxes Deferred tax assets and liabilities are recognized for the future tax consequences attributable to differences between financial statement carrying amounts of assets and liabilities and their respective tax bases, and for tax credits that are available to offset future taxable income. Deferred tax assets and liabilities are measured using enacted tax rates expected to apply to taxable income in the years in which those temporary differences are expected to be recovered or settled. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period the change is enacted
Loan Servicing	Loan Servicing Servicing rights are recognized when they are acquired through sale of loans. Capitalized servicing rights are reported in other assets and are amortized into non-interest income in proportion to, and over the period of, the estimated future net servicing income of the underlying financial assets. Servicing rights are evaluated for impairment based upon the fair value of the rights as compared to amortized cost. Impairment is determined by stratifying rights by predominant characteristics, such as interest rates and terms. Impairment is recognized through a valuation allowance for an individual stratum, to the extent that fair value is less than the capitalized amount for the stratum.
Post-Retirement Benefits	Post-Retirement Benefits The cost of providing post-retirement benefits is accrued during the active service period of the employee or director.
Earnings Per Share	Earnings Per Share Basic earnings per share data are based on the weighted average number of common shares outstanding during each year. Diluted earnings per share gives effect to restricted stock granted and stock options and warrants outstanding, determined by the treasury stock method.
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) includes net income and other comprehensive income (loss), which is comprised of the change in unrealized gains and losses on securities available for sale, net of tax, change in unrealized losses on securities transferred from available for sale to held to maturity, net of amortization, change in unrealized gain on cash flow hedging derivative instruments, net of tax, and unrecognized gains and losses related to post-retirement benefit costs, net of tax.
Segments	Segments The First Bancorp, Inc., through the branches of its subsidiary, First National Bank, provides a broad range of financial services to individuals and companies in coastal Maine. These services include demand, time, and savings deposits; lending; ATM processing; and investment management and trust services. Operations are managed and financial performance is evaluated on a corporate-wide basis. Accordingly, all of the Company's banking operations are considered by Management to be aggregated in one reportable operating segment.
Reclassifications	Reclassifications Certain items from prior years were reclassified in the financial statements to conform with the current year presentation. These do not have a material impact on the balance sheet or statement of income presentations.
Fair Value Measurement	Certain assets and liabilities are recorded at fair value to provide additional insight into the Company's quality of earnings. Some of these assets and liabilities are measured on a recurring basis while others are measured on a nonrecurring basis, with the determination based upon applicable existing accounting pronouncements. For example, securities available for sale are recorded at fair value on a recurring basis. Other assets, such as mortgage servicing rights, loans held for sale, and impaired loans, are recorded at fair value on a nonrecurring basis using the lower of cost or market methodology to determine impairment of individual assets. The Company groups assets and liabilities which are recorded at fair value in three levels, based on the markets in which the assets and liabilities are traded and the reliability of the assumptions used to determine fair value. A financial instrument's level within the fair value hierarchy is based on the lowest level of input that is significant to the fair value measurement (with level 1 considered highest and level 3 considered lowest). A brief description of each level follows. Level 1 – Valuation is based upon quoted prices for identical instruments in active markets. Level 2 – Valuation is based upon quoted prices for similar instruments in active markets, quoted prices for identical or similar instruments in markets that are not active, and model-based valuation techniques for which all significant assumptions are observable in the market. Level 3 – Valuation is generated from model-based techniques that use at least one significant assumption not observable in the market. These unobservable assumptions reflect estimates that market participants would use in pricing the asset or liability. Valuation includes use of discounted cash flow models and similar techniques.
Fair Value of Financial Instruments	The fair value methods and assumptions for the Company's financial instruments and other assets measured at fair value are set forth below. Cash, Cash Equivalents and Interest-Bearing Deposits in Other Banks The carrying values of cash equivalents, due from banks and federal funds sold approximate their relative fair values. As such, the Company classifies these financial instruments as Level 1. Investment Securities The fair values of investment securities are estimated by independent providers using a market approach with observable inputs, including matrix pricing and recent transactions. In obtaining such valuation information from third parties, the Company has evaluated their valuation methodologies used to develop the fair values in order to determine whether the valuations are representative of an exit price in the Company's principal markets. The Company's principal markets for its securities portfolios are the secondary institutional markets, with an exit price that is predominantly reflective of bid level pricing in those markets. Fair values are calculated based on the value of one unit without regard to any premium or discount that may result from concentrations of ownership of a financial instrument, possible tax ramifications, or estimated transaction costs. If these considerations had been incorporated into the fair value estimates, the aggregate fair value could have been changed. The carrying values of restricted equity securities approximate fair values. As such, the Company classifies investment securities as Level 2. Loans Held for Sale Loans held for sale are recorded at the lower of carrying value or market value. The fair value of mortgage loans held for sale is based on what secondary markets are currently offering for portfolios with similar characteristics. As such, the Company classifies mortgage loans held for sale as Level 2. Loans Fair values are estimated for portfolios of loans with similar financial characteristics. The fair values of performing loans are calculated by discounting scheduled cash flows through the estimated maturity using estimated market discount rates that reflect the credit and interest risk inherent in the loan. The estimates of maturity are based on the Company's historical experience with repayments for each loan classification, modified, as required, by an estimate of the effect of current economic and lending conditions, and the effects of estimated prepayments. Assumptions regarding credit risk, cash flows, and discount rates are judgmentally determined using available market information and specific borrower information. Management has made estimates of fair value using discount rates that it believes to be reasonable. However, because there is no market for many of these financial instruments, Management has no basis to determine whether the fair value presented above would be indicative of the value negotiated in an actual sale. As such, the Company classifies loans as Level 3, except for certain collateral-dependent impaired loans. Fair values of impaired loans are based on estimated cash flows and are discounted using a rate commensurate with the risk associated with the estimated cash flows, or if collateral dependent, discounted to the appraised value of the collateral as determined by reference to sale prices of similar properties, less costs to sell. As such, the Company classifies collateral dependent impaired loans for which a specific reserve results in a fair value measure as Level 2. All other impaired loans are classified as Level 3. Other Real Estate Owned Real estate acquired through foreclosure is initially recorded at fair value. The fair value of other real estate owned is based on property appraisals and an analysis of sales prices of similar properties currently available. As such, the Company records other real estate owned as nonrecurring Level 2. Mortgage Servicing Rights Mortgage servicing rights represent the value associated with servicing residential mortgage loans. Servicing assets and servicing liabilities are reported using the amortization method and compared to fair value for impairment. In evaluating the fair values of mortgage servicing rights, the Company obtains third party valuations based on loan level data including note rate, type and term of the underlying loans. As such, the Company classifies mortgage servicing rights as Level 2. Accrued Interest Receivable The fair value estimate of this financial instrument approximates the carrying value as this financial instrument has a short maturity. It is the Company's policy to stop accruing interest on loans for which it is probable that the interest is not collectible. Therefore, this financial instrument has been adjusted for estimated credit loss. As such, the Company classifies accrued interest receivable as Level 2. Deposits The fair value of deposits is based on the discounted value of contractual cash flows. The discount rate is estimated using the rates currently offered for deposits of similar remaining maturities. As such, the Company classifies deposits as Level 2. The fair value estimates do not include the benefit that results from the low-cost funding provided by the deposits compared to the cost of borrowing funds in the market. If that value were considered, the fair value of the Company's net assets could increase. Borrowed Funds The fair value of borrowed funds is based on the discounted value of contractual cash flows. The discount rate is estimated using the rates currently available for borrowings of similar remaining maturities. As such, the Company classifies borrowed funds as Level 2. Accrued Interest Payable The fair value estimate approximates the carrying amount as this financial instrument has a short maturity. As such, the Company classifies accrued interest payable as Level 2. Derivatives The fair value of interest rate swaps is determined using inputs that are observable in the market place obtained from third parties including yield curves, publicly available volatilities, and floating indexes and, accordingly, are classified as Level 2 inputs. The credit value adjustments associated with derivatives utilize Level 3 inputs, such as estimates of current credit spreads to evaluate the likelihood of default by the Company and its counterparties. As of December 31, 2016, the Company has assessed the significance of the impact of the credit valuation adjustments on the overall valuation of its derivative positions and has determined that the credit valuation adjustments are not significant to the overall valuation of its derivatives due to collateral postings. Off-Balance-Sheet Instruments Off-balance-sheet instruments include loan commitments. Fair values for loan commitments have not been presented as the future revenue derived from such financial instruments is not significant. Limitations Fair value estimates are made at a specific point in time, based on relevant market information and information about the financial instrument. These values do not reflect any premium or discount that could result from offering for sale at one time the Company's entire holdings of a particular financial instrument. Because no market exists for a significant portion of the Company's financial instruments, fair value estimates are based on Management's judgments regarding future expected loss experience, current economic conditions, risk characteristics of various financial instruments, and other factors. These estimates are subjective in nature and involve uncertainties and matters of significant judgment and therefore cannot be determined with precision. Changes in assumptions could significantly affect the estimates. Fair value estimates are based on existing on- and off-balance-sheet financial instruments without attempting to estimate the value of anticipated future business and the value of assets and liabilities that are not considered financial instruments. Other significant assets and liabilities that are not considered financial instruments include the deferred tax asset, premises and equipment, and other real estate owned. In addition, tax ramifications related to the realization of the unrealized gains and losses can have a significant effect on fair value estimates and have not been considered in any of the estimates.
New Accounting Pronouncements	New Accounting Pronouncements In January 2016, the FASB issued Accounting Standard Update (ASU) No. 2016-01, Financial Instruments - Overall: Recognition and Measurement of Financial Assets and Financial Liabilities. The ASU was issued to enhance the reporting model for financial instruments to provide users of financial statements with more decision-useful information. This ASU changes how entities account for equity investments that do not result in consolidation and are not accounted for under the equity method of accounting. The ASU also changes certain disclosure requirements and other aspects of U.S. GAAP, including a requirement for public business entities to use the exit price notion when measuring the fair value of financial instruments for disclosure purposes. The ASU is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. The ASU will not have a material effect on the Company's consolidated financial statements. In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842). The ASU was issued to increase transparency and comparability among organizations by recognizing lease assets and lease liabilities on the balance sheet and disclosing key information about leasing arrangements. The ASU is effective for annual periods, and interim periods within those annual periods, beginning after December 15, 2018. Management is reviewing the guidance in the ASU to determine whether it will have a material effect on the Company’s consolidated financial statements. In June 2016, the FASB issued ASU No. 2016-13, Financial Instruments-Credit Losses (Topic 326): Measurement of Credit Losses on Financial Instruments. Under the new guidance, which will replace the existing incurred loss model for recognizing credit losses, banks and other lending institutions will be required to recognize the full amount of expected credit losses. The new guidance, which is referred to as the current expected credit loss model, requires that expected credit losses for financial assets held at the reporting date that are accounted for at amortized cost be measured and recognized based on historical experience and current and reasonably supportable forecasted conditions to reflect the full amount of expected credit losses. A modified version of these requirements also applies to debt securities classified as available for sale. The ASU is effective for fiscal years beginning after December 15, 2019, including interim periods within those fiscal years. Early adoption is permitted for fiscal years beginning after December 15, 2018, including interim periods within such years. The Company is evaluating the potential impact of the ASU on its consolidated financial statements and anticipates the ASU may have a material impact.
Financial Derivative Instruments	As part of its overall asset and liability management strategy, the Company periodically uses derivative instruments to mitigate significant unplanned fluctuations in earnings and cash flows caused by interest rate volatility. The Company’s interest rate risk management strategy involves modifying the re-pricing characteristics of certain assets or liabilities so that changes in interest rates do not have a significant effect on net interest income. The Company recognizes its derivative instruments in the consolidated balance sheet at fair value. On the date the derivative instrument is entered into, the Company designates whether the derivative is part of a hedging relationship (i.e., cash flow or fair value hedge). The Company formally documents relationships between hedging instruments and hedged items, as well as its risk management objective and strategy for undertaking hedge transactions. The Company also assesses, both at the hedge’s inception and on an ongoing basis, whether the derivatives used in hedging transactions are highly effective in offsetting the changes in cash flows or fair values of hedged items. Changes in fair value of derivative instruments that are highly effective and qualify as cash flow hedges are recorded in other comprehensive income or loss. Any ineffective portion is recorded in earnings. The Company discontinues hedge accounting when it is determined that the derivative is no longer highly effective in offsetting changes of the hedged risk on the hedged item, or management determines that the designation of the derivative as a hedging instrument is no longer appropriate. In 2016, interest rate swaps were contracted to limit the Company’s exposure to rising interest rates on short-term liabilities indexed to one-month London Inter-bank Offered Rates (LIBOR). The interest rate swaps were designated as cash flow hedges.