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Benefit Plans - Defined Contribution Plan (Details) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 USD ($) date | Dec. 31, 2016 USD ($) | Dec. 31, 2015 USD ($)
Bank of Marin Employee Stock Ownership and Savings Plan (the Plan) |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Defined contribution plan, number of years to be fully vested | 5 years |  |
Cash contributed to the ESOP | $ 1,200 | $ 1,200 | $ 1,100
Defined Contribution Plan (the 401k Plan) |  |  |
Defined Contribution Plan Disclosure [Line Items] |  |  |
Minimum age of eligible employee for 401(k) plan | 18 years |  |
Minimum employment period to qualify for 401(k) plan | 90 days |  |
Number of open enrollment dates for 401(k) plan | date | 4 |  |
Minimum annual contribution per employee, percent of eligible compensation | 1.00% |  |
Maximum annual contribution per employee, percent of eligible compensation | 50.00% |  |
Annual vesting percentage | 20.00% |  |
Defined contribution plan, number of years to be fully vested | 5 years |  |
Employer matching contribution percentage | 70.00% | 60.00% |
Employer matching contribution maximum amount | $ 5 | $ 4 |
Employer contributions | $ 765 | $ 589 | $ 555

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- Definition Schedule of Defined Contribution Plans Disclosures [Line Items] + References No definition available. + Details
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- Definition Amount of cost for defined contribution plan. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 715 -SubTopic 70 -Section 50 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=49170846&loc=d3e28014-114942 + Details
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- Definition Percentage employer matches of the employee's percentage contribution matched. + References No definition available. + Details
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- Definition Percentage of employer's matching contributions to a defined contribution plan that vests in a given year. + References No definition available. + Details
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- Definition Maximum percentage of employee gross pay the employee may contribute to a defined contribution plan. + References No definition available. + Details
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- Definition The amount of plan compensation cost recognized during the period. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 40 -Section 50 -Paragraph 1 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=65895303&loc=d3e17540-113929 + Details
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