Basis of Presentation (Policies)	9 Months Ended
Basis of Presentation (Policies)	Sep. 30, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]
Basis of Presentation	The consolidated financial statements include the accounts of Bank of Marin Bancorp (“Bancorp”), a bank holding company, and its wholly-owned bank subsidiary, Bank of Marin (the “Bank”), a California state-chartered commercial bank. References to “we,” “our,” “us” mean the holding company and the Bank that are consolidated for financial reporting purposes. The accompanying unaudited consolidated interim financial statements have been prepared pursuant to the rules and regulations of the Securities and Exchange Commission ("SEC"). Certain information and note disclosures normally included in annual financial statements prepared in accordance with generally accepted accounting principles ("GAAP") have been condensed or omitted pursuant to those rules and regulations. Although we believe that the disclosures are adequate and the information presented is not misleading, we suggest that these interim financial statements be read in conjunction with the financial statements and the notes thereto included in our 2014 Annual Report on Form 10-K. In the opinion of Management, the unaudited consolidated financial statements reflect all adjustments which are necessary for a fair presentation of the consolidated financial position, the results of operations, changes in comprehensive income, changes in stockholders’ equity, and cash flows for the periods presented. All material intercompany transactions have been eliminated. The results of these interim periods may not be indicative of the results for the full year or for any other period. We have evaluated subsequent events through the date of filing with the SEC and have determined that there are no subsequent events that require additional recognition or disclosure. In connection with the acquisition of NorCal Community Bancorp ("NorCal") (the "Acquisition"), Bancorp assumed ownership of Norcal Community Bancorp Trusts I and II (the "Trusts"). Bancorp is not considered the primary beneficiary of the Trusts (variable interest entities), therefore the Trusts are not consolidated in our consolidated financial statements, but rather the subordinated debentures due to the Trusts are shown as liabilities on our consolidated statements of condition (see Note 6). Bancorp's investment in the common stock of the Trusts is accounted for under the equity method and is included in interest receivable and other assets on the consolidated statements of condition.
Earnings Per Share	Basic earnings per share (“EPS”) are calculated by dividing net income by the weighted average number of common shares outstanding during each period, excluding unvested restricted stock awards. Diluted EPS are calculated using the weighted average number of potentially dilutive common shares, which is based on average market prices during the three months of the reporting period, under the treasury stock method. The number of potentially dilutive common shares included in year-to-date diluted EPS is a year-to-date weighted average of potentially dilutive common shares included in each quarterly diluted EPS computation. We have two forms of outstanding stock: common stock and unvested restricted stock awards. Holders of unvested restricted stock awards receive non-forfeitable dividends at the same rate as common shareholders and they both share equally in undistributed earnings. Therefore, under the two-class method, the difference in EPS is not significant for these participating securities.
Recently Issued Accounting Standards	In May 2014, the Financial Accounting Standards Board (FASB) issued Accounting Standards Update (ASU) No. 2014-09, Revenue from Contracts with Customers (Topic 606). The ASU is a converged standard involving FASB and International Financial Reporting Standards that provides a single comprehensive revenue recognition model for all contracts with customers across transactions and industries. The core principal of the guidance is that an entity should recognize revenue to depict the transfer of promised goods or services to customers in an amount and at a time that reflects the consideration to which the entity expects to be entitled in exchange for those goods or services. In August 2015, the FASB issued ASU No. 2015-14, Revenue from Contracts with Customers (Topic 606) Deferral of the Effective Date which institutes a one-year deferral of the effective date of this amendment to annual reporting periods beginning after December 15, 2017. We are currently evaluating the provisions of this ASU and will be monitoring developments and additional guidance to determine the potential impact the new standard will have on our financial condition and results of operations. In June 2014, the FASB issued ASU No. 2014-12, Compensation - Stock Compensation (Topic 718) Accounting for Share-Based Payments When the Terms of an Award Provide That a Performance Target Could Be Achieved after the Requisite Service Period. This ASU provides guidance for entities that grant their employees share-based payment awards where a performance target that affects vesting could be achieved after the requisite service period. That is the case when an employee is eligible to retire or otherwise terminate employment before the end of the period in which a performance target could be achieved and still be eligible to vest in the award if and when the performance target is achieved. This ASU stipulates that compensation expense should be recognized in the period where the performance target becomes probable of being achieved as opposed to the date that the award was granted. ASU 2014-12 is effective for annual periods and interim periods within those annual periods beginning after December 15, 2015. We do not expect this ASU to have a material impact on our financial condition or results of operations. In April 2015, the FASB issued ASU No. 2015-05, Intangibles - Goodwill and Other - Internal-Use Software (Subtopic 350-40) Customer's Accounting for Fees Paid in a Cloud Computing Arrangement. The amendments in this ASU provide guidance about a customer's accounting for fees paid in a cloud computing arrangement. If a cloud computing arrangement includes a software license, then the customer should account for the software license element of the arrangement consistent with the acquisition of other software licenses. If a cloud computing arrangement does not include a software license, then the customer should account for the arrangement as a service contract. The two criteria that must be met to be considered a software license are: 1) the customer has the contractual right to take possession of the software at any time during the hosting period without significant penalty; and 2) it is feasible for the customer to either run the software on its own hardware or contract with another party unrelated to the vendor to host the software. ASU 2015-05 is effective for annual periods, including interim periods within those annual periods beginning after December 15, 2015. We do not expect this ASU to have a material impact on our financial condition or results of operations. In September 2015, the FASB issued ASU No. 2015-16, Business Combinations (Topic 805) Simplifying the Accounting for Measurement-Period Adjustments. The amendments in this ASU require that an acquirer recognize adjustments to estimated amounts that are identified during the measurement period in the reporting period in which the adjustment amounts are determined. Prior to the amendments, an acquirer was required to retrospectively adjust the provisional amounts recognized at the acquisition date with a corresponding adjustment to goodwill. ASU 2015-16 is effective for public business entities for fiscal years beginning after December 15, 2015, including interim periods within those fiscal years. We do not expect this ASU to have a material impact on our financial condition or results of operations.